Here is a small cymbal breakdown from "best" to "worst":Thanks! I hadn't even realised Gumtree was in the UK (for some reason I assumed it was like Craigslist and US only). Still nervy about buying something I don't even pretend to understand properly, but would be good to be able to have some metal to hit when santa arrives etc.
B20 bronze: Zildjian K and A, Sabian AA and HH
B8 bronze: Zildjian ZBT, Sabian B8 and B8 Pro
Nickel Silver: Zildjian Planet Z
Brass: Stay away from brass!
There are plenty of other manufacturers besides what is listed that make quality cymbals. The key words you need to look for are B20 or B8. Do some homework, there is waaaay too much to list and explain here.
Things to look for when buying used:
Keyholes: when the cymbal mounting hole is worn in one particular spot. It looks like a small hole on the edge of the mounting holes.
Fleabites: a small nick on the edge of the cymbal. It may be nothing, it may turn into a crack
Cracks: self explanatory, these are broken and can't really be fixed. They won't sound good.
Keyholes and fleabites don't necessarily affect the sound of the cymbal, but they do devalue it some and make a good negotiating point.