1st thing I did last winter is contacted my accountant and got a plan together. I contacted the HD with my ideas and they sent me in the right direction. My HD required I get Serve Safe certified, which I did.
My cooker was NSF already and I had most all the small stuff I needed. I was able to use a certified kitchen to do my prep work and use their coolers. I hooked up with a meat supplier that let me keep my meats and salads in his coolers till I needed them.
We hit the ground running and had a great 1st year. 4 weddings and a bunch of smaller catering events. We got a small vending trailer and did our county fair and a few other smaller events in the area. I ended up having to buy a new truck so my tax exposure won't be too bad. By mid-summer I bought another cooker to keep up with the volume and be a backup just in case. That was a great idea because I got a bad batch of pellets and it took my original one out for a few weeks.
I know this is not typical but it shows what can happen if you play your cards right and get really lucky.
Good luck on your venture, don't let anyone tell you it can't be done.
It's a huge amount of work but all worth it when people tell you how good you food is.