Well, make sure you have everything inspected by a trusted mechanic if buying from a private seller. If buying from dealer then make sure it comes with some sort of warranties.
That year/model BMW is particularly problematic with its transmission, the no reverse issue (if automatic); depends on mileage, there could be other issues with its cooling system, VANOS, bushings, interior trims, etc...
If you can get a VIN number, go to realoem.com and find out exactly what that car has, when it was built. Carfax is something you should invest in as well regardless of where you're purchasing this from. If you have any particular questions, just PM me as I know a bit about these cars mechanically.