Welcome Skydrifter! Sounds like you've a got a good head about you. I wouldn't go too old on your TT. Maybe a year or two. Just long enough for the previous owner to work out all the first year kinks, but still has a lot of life to it.
I run a 25'r and am very happy with it. Plenty of room, but still small enough to get into just about anywhere. I'll second the post 08' and the 6 speed transmission. Makes a world of difference when towing.
20k for a tow rig might be a little shy. It's doable, but your probably going to be looking at some pretty heafty mileage.
Welcome to the club!