I am going to say, features to look for:
As Frank mentioned, an Aluminum Frame is a plus because they are allot lighter.
Urethane bladder is most desired because it is lighter, last allot longer, less effected by Altitude and temps. Vinyl is okay and is going to be pretty much what you will be looking at in the lower price range. Just KNOW that folding vinyl can cause holes, and be sure and transport half full because it will balloon up at higher elevations. Hot days, inflate then put in the cold water for several minutes then check the air again.
Like a hot dog, the outer cover is the most important part. Get a heavy denier. Plus a rhino hide bottom is always a plus.
Seat, out of all the ones out there, I am going to say the PADDED molded plastic seats are the best.
I would recommend to you an Bucks Bag Alpine. The new ones are Vinyl, but a dual air chamber. 7' long and low profile. Small enough to fit in the back of a small pick-up
A little more money, but worth it to me: