TS&M pretty much nailed it.
If it does not fit you, you will never be comfortable in it and you will never get good with it.
It needs to be large enough to give you full range of motion without being baggy. Too large means too baggy, too much drag and lots of potential problems controlling the air volume in it.
Boot size needs to be in the ball park as well. Too small and your feet cramp, to large and your feet could come right out of the boots - especially if the legs are also a bit long. It is not always a deal killer as boots can be replaced on the suit, but it is a $200-$250 expense to change them.
Zippers are in the $125-$175 range just for the zipper and another $150-$175 in labor is not out of line to install a new one.
Wrist seals are comparatively cheap to change and pretty easy to do yourself. Neck seals are a lot harder and a lot less forgiving. Sending it out to have the neck seal repalced is probably going to be in the $75-$100 range plus shipping.
In addition to changing zipper, seals, boots, etc, Superior divers will do tailoring on a dry suit, making things longer, shorter, thinner, wider, etc, but the costs add up so if you get a great deal on a less than perfect suit, it may still make sense, but get quotes on the needed alterations in advance to avid paying a near new price for a used suit.
Aquaseal around seams can indicate a seam leak, but if the stitching is intact and the aquaseal repair is smoothly applied, it is not a deal breaker - it may just indicate the suit did not fit the old owner very well (likely the case if the seam leak is in the crotch area - an area easily stressed if the torso is too short (usually meaning the diver's torso was a bit too "round").
Fabric is important. I'll spend money on a good used tri-lam (nylon outer layer, rubber center layer and a nylon or polyester inner layer) but will pay nothing for a bi-lam (shiny latex or polyethelene inner layer and a nylon outer layer. Once the inner sealing layer starts breaking down, the suit leaks like a sieve and there is nothing you can do to salvage it.
Pockets are a fairly easy DIY project. You just need a pocket(s $30 to $80 eache depending on brand and how good the deal is) some Aqualseal and cotal 240 to glue it on ($15-$25) and some duct tape to outlnine the area to be glued to keep things neat and clean looking.
Undergarments are expensive - the dry suit may no longer be a great deal if they are not included and you have to spend $175-$350 on a new set.