If you are buying used---you are looking for cracks or other damage, and as far as buying new the only real issue is do you like the sound. I was told by a drummer friend when I first started building my kit to buy a pre-selected set to start with, and add or replace individual cymbals over time. I currently have Zildjian
14" hats, 16" crash, 20" ride, 18"china, and a sabian
18" crash/ride. I bought the sabian
new to upgrade the cheap ones that came with the set, then got a great deal on the zildjians
used. One thing to keep in mind is that cymbals will sound different at the music store than they will in your living room!