If you're not mechanically savvy I would get a mechanic to check it over. Or if you don't know a mechanic get the AA or RAC
http://www.theaa.com/motoring-advice/vehicle-inspections.html
http://www.rac.co.uk/products-services/vehicle-inspections/
I used the AA myself. They seemed quite comprehensive. It's a small price to pay when you consider any "big" job on the car will cost you £1000 plus...
You really want to know about any jobs that are going to cost you big $$$$ to get fixed. Any old-ish car you buy is going to have some problems, so you should budget for that, but hopefully the problems are just smaller things that you can quite cheaply fix.
Look at it this way. If the AA cost you £200 to get it checked out. You could get four cars checked out for the price any medium\big job on a dodgy car is going to cost you i.e. £1000. Just look at it as an investment\risk.
The bigger things I'd be interested in are things like the following, all of which you could probably say about most cars...
- Is the turbo on the way out. Could cost you the best part of £1000 to get fixed
- Is the gearbox\transmission dodgy. Could cost you from £1000 up to a lot more!
- Is the clutch or flywheel on the way out. Will cost you the best part of £1000 to get fixed
- Any suspension or steering related problems. Could cost you from £500 upwards...
- Is the engine mechanically sound
- Has the car been in a crash...could end up costing you the majority of your investment!
Obviously checking that the required services are done on things like oil\filter change, haldex oil\filter change, brake fluid, timing belt etc. will all give you more confidence in buying the car and reduce the chances of it being a dodgy car.
As for timing belt, regardless of when it was done I would get it done immediately after buying it myself. Do you really trust a service history? Would you be surprised if the previous owner somehow managed to get a stamp without getting the work done? How would you feel if you drove the car for a week, timing belt snapped, and you engine is left in bits, then you find out the timing belt that was in looked a lot older and worn than you thought?
There's a risk in buying any second hand car. All you can really do is reduce the risk by doing things like I've said above. Personally I would do the following:
1) Check the service history and everything about the car yourself. Once you are happy, then...
2) Get a mechanic or the AA\RAC to check the car. If they give it the all cear, then..
3) Buy the car, get it a full service, and get the cambelt\waterpump changed straight away