Re: Building a food trailer/truck.
Build or Buy: Do you have the space, tools, and skill to do a build? Do you have any help? You'll need to do alots of trade skill tasks potentially including metal work, electrical, plumbing, gas lines, etc. You'll need knowledge in many areas or help in those you don't have.
Do you have the time? You can get up and running a lot faster buying a trailer.
Truck or trailer. A Truck will cost a lot more to buy than a trailer, but if you're building from the ground up, a truck may not be quite as much more. For example: You can buy a full trailer for say $10k-30k and Buying a truck will cost $40 if you're REALLY LUCKY, but more like $50-80 for a used one. But building from scratch, the basic trailer is $2-5k plus build and equipment cost and the truck can be anywhere from $5-$25k plus build and equipment. So if you can score a decent truck for under $10k, your build cost for a truck are not astronomically higher than a trailer, but it is more complicated (do you have auto skills as well?)
A truck can go more places more practically, but if you're looking at doing part time at events and festivals, a trailer is not a big limitation. if you're looking at full time daily operation, a trailer will be difficult.
So in summary: How do you want to use it? How much can you spend in time and money? And what are your skills?