Thinking back when buying mine most common faults:
- Check the clutch bite, although some people will say that a high biting point can be normal, from the Zeds I drove most are not high and this can be a sign of a clutch on the way out, £1000 for a new clutch kit and fitting. Its just not worth the risk if you are on a budget, look for one with a replacement.
- Check the windows, motors are prone to playing up, sometimes its a failed motor sometimes it just needs resetting but worth checking.
- Check underneath for any wet oil, could be a sign of a leak, at that mileage could be an issue. Make sure they guy starts it up from cold and stand at the back when he does to check for any blue smoke out the exhaust. If he hasn't driven it for a while there might be a squeak of it, but if a fairly large plume comes out you know there are problems.
- Otherwise the advice above is good, check suspension, check for paintwork problems.