Normally I would volunteer to help - but my weekends are tied up at the moment.
Price: Have a good look through Autoscout - you'll be able to get a good idea of whether a car is priced well or not.
Condition: If the car has a new MFK these is a good chance that all the important mechanical bits work. However that doesn't mean the air con, the stereo, the head gasket and the gearbox won't let go within a week. The service history and receipts will tell you as much about a car as anything.
Length of Sale: How long has the car been on Autoscout - does it get relisted?
I have had very positive experiences of buying from mechanics who also sell cars - compared to car dealers. I can recommend a couple - but neither are in Zurich.
1) Take the car along a badly surfaced road - listen out for clunks. This may also happen on full lock. Problem can be ball joints, CVs
2) Check tyre wear - if uneven it can be down to badly inflated tyres, enthusiastic driving, wheel alignment - or damage to the suspension eg bent wishbone
3) Braking - do the brakes squeal (often down to non-standard pads) - does the car brake evenly? or pull to one side? What is the wear like on the discs? Is there a big lip from the disc face to the edge?
4) Cambelt (Zahnriemen) - dig about in the service book - find out when it should be done? and when it was last done? While you are at when was the last service done? And what sort of service? And what else was replaced? Most mechanics will write ERZ Wasser pumpe - which would mean replaced the water pump.
5) Model specific problems - get onto google - and look up the car for problems. Parkers.co.uk HonestJohn and owner forums should help.
6) Coolant problems - look at the radiator - does it have a lot of stone chips or blown fins - look in the coolant expansion tank - is there a white gunk?
7) Listen and look - when you start the car up does it sound right? do the panels look right? does the paint finish look right - look along the panels not at them. Does the engine pull smoothly in all gears? At low revs? Does the clutch slip? Are there strange smells?