Believe it or not you can find some pretty good used trailers on e-bay. I bought mine off ebay. You may have to travel to get it, but it'll save you a good chunk of change. Some of the ones I looked at were pretty built-out with cabinets, nice floors, etc. With all the stuff you want, you'll probably wind up putting at least some of it in yourself. A twenty footer is going to get cramped with everything you want. You may want to look at a 24' or 26'. My 26' weighs in around 4000 empty. I like the inflatable matress idea over putting fold-up beds on the wall.

My guess would be $12-14k for a new 20' with the stuff you want installed. That's just a SWAG based on what I remember about the prices I saw on e-bay. Could probably save some money if you just get a base trailer and install stuff yourself. You didn't say anything about flooring or walls and that stuff can add a good amount to the price as well.

David