A few years ago, I bought a 3 horse gooseneck trailer that was just the shell. No fanciness in it. That winter it was a good project for us finishing it all up. We made it into the "weekender package". It has a sink, fridge, futon, A/C, hot water heater, a combo shower/toilet, and cabinets throughout. Doing it ourselves saved a ton of money. If we would have bought it with it done, it would have cost us $8,000 more than what it cost us to make it. Plus it was pretty fun. I felt like a mini interior designer for a winter. :) You just have to make friends with a guy that works for a camper/motor home business who is willing to order stuff. Some of the water/waste tanks, insulation (it's strange stuff), and fabric that is used on the walls is kind of hard to find otherwise. As far as length, mine just has a 8 foot shortwall yet I have all the room I could ever need. Makes it easy for me to pull (and back up - yay!).
When I go to a show, I spend 4 days there so for me it was worth the money used to finish it out, considering what I would be spending on hotels and the luxury of getting to stay right where my horse is at. If you aren't going to be sleeping in it regularly, it might not be worth it, but I think A/C is very necessary no matter what you're going to use your trailer for.