David,
The thoughts you are describing are exactly the thoughts I've been having. I'd like to cook and sell.
But from there, things got complicated quickly. And I'm not talking about gov'mt regs.
From my wife's point of view, how could we do this with a minimal investment? She's concerned I might see what the work for this business realy takes, then back out, after putting in serious $'s.
This one is REAL tough. I'm fighting the same battle. We have NO cash flow right now -- we're paying bills fine and living OK but not much extra "start up money". I'm working towards getting the rig and then fighting the rest of the battle.
So, do we do a trailer smoker and corner arrangement like you describe? If so then, this would be like a community BBQ vendor, ready made meals type thing. It would be desireable to have folks call in in advance to help know how much meat to cook. This is especially true for brisket, since it takes such a long time to cook. Call it a "reservation".
The advantage to the "street corner" vendor (for me it can be year round - I live in FL) is that I have no real estate overhead other than lot rent to the owner of the building I'm parked in front of. You'd want to find your semi-permanent place -- you will get regulars that way. Now, if you're doing this there is no guarantee -- cook and hope they show up just like a brick-mortar restaurant. I'd start with a couple dozen chickens and a case of ribs -- when I sell out I'm out and cook more tomorrow, if needed -- all this to try out the market. Catering on the other hand is figured towards your profit per job -- like someone else has said you can get about 50% ahead deposit. However, you've still got to beat the bushes for customers -- so, back to the road-side/parking lot stand -- gotta get word out about your product.
Then what about overage? Can any overage be saved and used the following day, with no loss in quality?
This is entirely up to you -- yes you can save your product - in the cafeteria business we called in "run-out" i.e. it's what you "run out" first on the next day. With chicken I'd bone it and make chicken salad or soup - or just donate it to a shelter. Pulled pork would hold, so would brisket, maybe ribs - warm 'em up slow the next day. This is entirely up to you -- best bet would be cook what you think you'll sell and if you run out -- go home
Smoker? I'd have to make an investment in a trailer smoker. Which is fine, but is it suited to a street corner operation? What about nights? In other words, someone could hitch up and drive off with the smoker, overnight. But if I trailer it home, I might have to be cooking the brisket on it to have ready for the next day, neighbors might object to that after a while. Good news is, I think the Health reqs. are less for a corner type operation.
These are decisions ONLY YOU can make - I'd never leave my rig unattended. I'd plan on finding a secure strorage site (like RV storage place) and pay the rent. Regs are similar but you have to have a vendors permit (in most places) and they could still ask about your commissary kitchen.
Then, is it a one person operation? If so, you need a smoker that runs with minimal babysitting. You can't babysit a smoker and tend to customers. Then there's side orders, what would they be? What if you run out? I myself don't like to be told "sorry, out of (what you ordered)". What if there's too much? Donate to needy?
I'd figure on a one-man operation to start. Hopefully the wife would help sometimes. Keep it where you can handle it with minimal support until you get that big gig where you can "hire" a couple of teenagers to fill coolers and spoon up cole slaw. Most any of the big rigs run with minimal supervision (I know D. Klose's rigs and Langs, etc. don't need a lot of tending once you get 'em tuned and your fire up to temp. Your food should just about be ready before you're dealing with customers. My sides would be cole slaw or potato salad, maybe beans, some kind of wrapped dessert (like individual pecan pies, or such). These would be alread portions in covered plastic or styro takeout cups. If you run out you run out!! If your meats good they'll be back
Sides are really incidental to the meat -- you'll need to price your meat by the 1/2 and full pound, too. You'll have people want ribs, chicken, pork, or beef to take home -- trust me!! Yeah, leftovers are an issue - you're salads and bean could be held one day (rule at the cafeteria was one "reheat" or back second day and then out -- it could be donated to a shelter. Just make damn sure you're giving them good stuff - it may be a day old but it's still safe and tasty.
Advertising... I'd want to snail mail notices to the surrounding neighborhoods/homes to let people know I existed, with a map, and to provide the phone number for advance orders.
Word of mouth and some printed flyer will get you started. Free food to the DJs at a couple of local radio stations wouldn't hurt. Do up a catering flyer and hit churches, offices, fraternal halls, etc. Try and get a couple of small gigs to try everything out. Don't be afraid of events that aren't necesarily 'Q -- burgers and dogs for 100 kids will pay the bills.
Is there any profit in it? If I do all this, cook, and sell, is it profitable enough given the effort involved? How do I go about doing a business plan with the calc's to determine this?
Hell yes there's profit! I figure my Saturday gig would have NETTED me over $300 -- that's NET. Dessert Sunday would have netted me probably another $150 or so - I'd alread shopped and all I did was make coffee, tea, lemonade, and plate up the stuff for buffet service. Not bad for about 10 hours work including my shopping. You've got to price yourself competitively -- too often startup caterers try and go with cheaper prices -- you'll lose you ASS! Find out what the local barbeque joints charge or talk to other caterers (out of your local area -- so they won't think you're moving in on their territory). The good ones and those that stay around are not cheap. Let your food speak for itself. Be prepared to cater full service -- you provide food, plates, cutlery, table cloths, tables (if needed), serving and cleanup. Also be ready for product only gigs -- you deliver the food and leave. And combinations. Never turn a gig down!! Find a way to make it work or refer to another caterer -- paybacks are good!!
Would I have to make my product and therefore myself available when everyone else is "off", which is nights and weekends? Would I be an "absent" parent? Would I be tending the business so much, would my wife and kids still know me?
It's like any other business -- depending on how you run it it can eat up all your time. Or, you can involve the family. Or, do just enough business to pay the bill and take the family camping. One of the 'que joints I go to here in Florida is only open from after Labor Day until just after Memorial Day each year - they are only open Thursday-Saturday - they are in a screen shack with no air conditioning and you sit near the pits - and they area ALWAYS FULL -- it's the que and nothing else that brings peopl in -- they've been doing business like that since at least 1966 when I started making road trips for their ribs

So, only you can answer the bulk of this one -- how much time will you put in?? I don't know - a lot depends on your market and how much you love to cook and serve food. I love it and am doing IT work because I was ready to do harm to my last Associate Manager

I get a lot more time with the family now and the money is OK but I'm not doing what I love to do. My wife and family fully supported my decision when I left IT for the food business - they put up with the 60-80 hour weeks - but we were a two meal a day, 7 days a week, commercial cafeteria. If I get into the 'que business it's going to be on my terms - for a change.
This list of thoughts and concerns is partial, but it gives you an idea of what I'm thinking about doing, and what my wife can "get on board" with. She has started her own small business, and knows how to do the financals and the necessary filings and paperwork.
Get a couple of books like "Start & Run a Catering Business" by George Erdosh. There are others -- check Amazon.com or Barnes & Noble, Books-a-Million, etc. Gather all the info you can. Try to get some experience with large group food prep even if it's not 'que. 
Good Luck!! I'm trying to get started myself -- the only "advantage" I've got is that I've had my ass whipped in the cafeteria business and nothing about this scares me (makes me careful, yes!!). To coin a phrase that we used often when a bus pulled in in the middle of a rush, "I ain't skeered!"

I also understand food costs and purchasing for large group feedings -- when I finally start up it'll still be on a wing and a prayer 'cause I don't have any deep-pocket sponsors, either.
-egkor (Gary K)