Well, a gently used cargo trailer can be converted to a decent, though limited, catering kitchen.
You didn't say where you are located, but most states require a lot to do a mobile kitchen, but are somewhat more flexible if you're just looking at cooking meat and keep the side items simple or better yet, store bought. In most states, there is an exemption for site cooked food, as this is viewed as more of a personal chef type deal. Most festivals, fairs, and BBQ competitions will have requirements as to the way food is held, and some will be strict.
I'm in the process now of converting an 8x18 cargo trailer into a catering rig. I have no intention of going all out and getting it approved, but safety and sanitation are my primary concerns. I'm re-sheeting the interior walls with FRP wallboard and doing solid vinyl on the floor with all the corner seams sealed with caulk. I've added light and plan to do an A/C unit too. I don't plan to cook in the trailer, so I won't need a hood, but I do plan to prep food, with the smokers just outside.
In my case, I'm going with a two door reach-in, a small freezer, a triple sink, a prep table, and a hand sink mounted near the door so I can just reach in and wash my hands. A fresh water and grey water holding tank and small water heater round out everything I need.
If you already have the trailer, you can do everything I just listed for about $4K if you shop wisely via the internet. I ended up paying $792 delivered for the triple sink, where I had been quoted $3K from my local supply store.
I just bought a whole pig cooker which I'm having mounted on the back of an open motorcycle trailer that has had the rails removed. I'm doing a grill on one side, fryer on the other, and room for an Amerique up front. The whole thing will roll right up into the larger trailer(mother ship) with my generator in back. I can be fully set up and cooking in 10 minutes, or I can just pull the smaller trailer if that's all that I need.
Didn't mean to run on. If all you want is a hand sink, Cambro offers a nice portable unit
Cambro Hand SinkThis company offers a large selection too.
Portable Sink.comOr you can do what a lot of folks do, and get a mop basin from Lowe's and a couple of plastic drums and make your own system. It might not be pretty, but it'll work and be cheap. It all depends on what you want and need to do, plus are required to do to be safe and legal.
Pellet Envy has some nice rigs. I especially like the dual FEC100's. It's only partially enclosed, but it looks great.