The things I would look for before buying a drysuit are:
1) suspenders
2) front entry zipper
3) attached boots
4) perfect fit
Some of the bargain suits do not have suspenders, so they always slip down when you try to wear them half-on for coolness before the dive.
I have no idea who invented back entry zippers, but they were a bad idea from day one. Sure, someone can zip it for you, but you will never know if they Did It Right. The diagonal front zippers are easy to close yourself. At least, I have never had a problem.
If the boots are not attached, then that is two more things you need to do when gearing up in the suit. Some people love their rock boots, but I don't. I have never had to wade over rocks in a drysuit.
For perfect fit, I would always specially order a drysuit. If you take the measurements and find the stock suit that is supposed to fit you, then there usually is no extra charge, you just have to wait. Waiting for a perfectly fitted drysuit is well worth the wait.
The great thing about Ebay is that you can always sell your drysuit to someone, for the right price, and get a better one the next time.
Uncle Pug recently did a great all-things-considered posting for drysuit choices. I forgot where it is, or I would cross reference to it. He talks about zipper choices. Excellently!