Pick up a copy of The Complete Trailer Sailor by Brian Gilbert. He has tons of things on trailering, sailing, launching and the like. But he also has a section in the back that has 50 or so sailboats from 16' to 28'. He includes photos, sketches and specs so you can decide what is right for you. One big thing in my mind is . . . where's the head. Most of the trailerables place the porta-potti in the middle of the cabin floor for all to enjoy. Not for the likes of me but fine for others. Head room is another issue to keep in mind.
Don't limit yourself to a 20 footer. It's a sizable boat for sure but if you go just a little larger, you can get twice the boat and if the you shop a little (especially in the fall) you can get great deals. We bought our Lancer 25 on CraigsList in November of '08. She was completely seaworthy but in the past few years we have upgraded this and improved that. We trailer her every time we sail so we've gotten pretty good at that. And stepping the mast only takes about 15 minutes with the two of us. We just got back from a 6 day excursion up the Maine Coast.
Best of luck!
Biddeford, Maine USA