They are a fantastic little car. Unreal to drive and would shame an awful lot of other cars on the road. Few things from my experience to look out for.
- cooling problems. Make sure to get to start the car from dead cold, many of them are missing a heat shield between the coolant bottle and exhaust manifold. If it's driven any length of time with it missing you could be looking at headgasket problems.
- access in the engine bay is extremely tight even changing aux belt is a complete pain.
- I've heard a few comments about the earlier models having gearbox problems don't quote me though.
- power steering can be quite troublesome, listen for any wissing noises while going on full lock and steering feel going from heavy to light or vise versa. This is a costly fix

However i found them quite easy to insure and are very easy on fuel. Handy to tax too which is rare for a BMW just my two cents.