(Original post by 1992LP)
Basic things to know about buying a used car:
Buy on condition, not mileage. Average mileage a year is 10 - 12k. Low mileage doesn't always mean better car!
Service history is very important, oil should be changed once a year, not by mileage. E.g. a 2002 car with 50k on it should have had 10 services, not 5.
Try to buy a car with a full MOT, and check the advisories. Long tax is always a plus, and can be used as a bargaining point.
Check all the levels (oil, coolant etc) under the bonnet, and look under the oil filler cap. It should be clear and free from sludge.
Check all the tyres for uneven wear, and just wear in general. Also try to avoid cars with crap tyres on them (i.e. £20 specials) as they don't grip well in the wet and is a sign that the last owner has skimped on maintenance. Can also be used as a bargaining point though.
Listen for knocks and bangs on the test drive, and pay attention to the clutch - does it slip? How high up the pedal is the biting point?
Look for smoke - blue smoke is oil, white is water, and black is fuel. Black soot is normal for diesel cars without a diesel particulate filter.
Check for rust in the wheel arches and under the boot carpet. Look at the panel gaps - if they are uneven then the car may have been in a crash at some point. Also look for mis matched colour and overspray as well.
Then look for the common problems associated with the car you are looking at - www.honestjohn.co.uk
is good for highlighting these, and so are some of us on here!
An old Micra (the K11 is the one you want) is a fairly safe bet. Very reliable, very cheap, and cheap to fix if it goes wrong. At £350 condition is everything. If the car is tidy and drives fine then it is not the end of the world if the service history is missing.
Hope that helps you a bit - any more questions, fire away