Before you buy anything:
If you haven't already, you need to contact your local health department to see if what you are planning is possible (legal) from their end and what their requirements are. You'll likely need to check with the fire department to see what their requirements are, and the various county offices that regulate/license businesses. See
this post and cover all the bases, informationally, first. Be prepared to ask questions and respond to theirs, especially the HD's, in regards to on-site or off-site food prep and storage, hot-holding and cold-holding, cold storage, water availability, required sink type(s) and quantity(ies), and so forth. See also
this post.
If the overall project is acceptable to the county, their requirements might very well steer you toward one type of trailer over another. E.g., on-site cold- or hot-holding requirements might mean
this sort of trailer set-up rather than
that; ditto with sink requirements, among other possibilities. Cooker size might be rather immaterial compared to other requirements. It all depends on the requirements. Some counties have so many that it is prohibitive; others--sometimes counties immediately adjacent--have far fewer regulations. It can be worth checking other counties nearby.
Used trailers are posted on various Q websites. There are many manufacturers. A Google search for
'barbecue concession trailer' will show many. Some carry used/trade-ins. Ebay lists new and used as well.
Some counties might offer a catering license or concession license for what you are looking to do--but not if your set-up is 'permanent', i.e., in the same location all the time. Find that out. Some will not allow you to use your set-up if, someday, you decide to open an actual joint. In that case, buying larger now is unwarranted. You'd need a different set-up for a joint--but could either sell the trailer or keep it and use for off-site catering gigs (many joint operators keep a trailer for this very thing). Again, asking questions now, as you form plans, can help immensely in the long run. Also, cold storage and hot-holding can have much to do with what type of trailer you need. More than a few caterers cook much of their food ahead of time and reheat on site, allowing them to operate with a smaller set-up. Lots of possibilities trailer-wise...
