"catering" with a WSM is more like "personal chef'ing". To paraphrased "Jaws": You're gonna need a bigger boat (cooker).
I don't cater now but when I did I used the heck out of my Kingfisher stickburner to turn out port for 200-400 and ribs, brisket, chicken, etc. for Superbowl parties, smaller church functions, business holiday parties, birthday parties, etc. I can't imagine trying to go bit without the proper equipment.
Also, simple things like tongs, aprons, chafing pans/stands, gallons of rub and sauce, TIME (never enough), dealing with weather and customer personalities...it's indeed a job. As was said earlier: check out how many friends and family will pay what your work is worth - and always, always, always, get enough deposit to pay for any up front costs before you start. Cash the check at the customer's bank - do not deposit to your account and then find out it bounced! It'll ruin your day and credit rating.
Good luck! It's fun but REAL HARD WORK!