Take a good look at one of the pre-purchase checklists for 2002 online (here on the FAQ and elsewhere.)
Here is one of the better ones that I have found. Suggest printing it out and bring it with you:
http://laudeman.com/bmw_checklist.html
If you can swing it, a PPI (pre purchase inspection) by a mechanic who knows BMWs is a good idea. Most of the better independent BMW shops around the country will do PPIs for a reasonable fee (I haven't priced it lately but I'd say around $150). Well worth the investment.
If you are thinking about buying a car over the internet, sight unseen, a PPI by a local mechanic (in the area the car is, obviously), that you pay for, is your best and only insurance.
Finally, if you are looking for your first 2002, definitely get a book. The Mike Macartney book "02 Restoration" has been well thought of. It goes into the rust areas (including replacement of the sheet metal) in tons of detail. You can find it at many booksellers online... even at $20 it is a drop in the bucket compared to a $1500 car (heck, if you buy a perfect $12,000 car you might not need it, but on a $1,500 car with needs, you're much more likely to.)
BMW '02 Restoration Guide (Brooklyns Restoration) by Mike Macartney
BMW 1602 and 2002, 1959-77 (Haynes Manuals) by John Haynes
BMW 2002: A Comprehensive Guide to the Classic Sporting Saloon by R.M. Clarke