Well first of all, it mainly depends on what you want to spend on the car and how much you are buying it for...for less money more problems are acceptable.
The 15k rule is close. Personally I don't mind maintainance issues because I plan on keeping my car forever and spending lots of money to keep it in tip top shape...so some of the chronic problems like worn alternator bushings, buzzy shifter, rear interior panels coming off, airbox mount coming off, leaking intake manifold gaskets, intermittant instrument cluster gauges, etc...would not deter me from a purchase.
I would stay away from cars that have more serious problems like rust, whether it be on the metal below the windshield, below the rear window, by the license plate lights, on the firewall. I would also stay away from any car that has or may have frame damage (or maybe any body work).
It really depends on what you want to put into it though....with the M3 I don't mind putting money into it so more things are acceptable...maybe even a blown motor because I wouldn't mind rebuilding it to a 2.5. However if you are looking for a pristine weekend car and low-maintainance DD, you want to spend more money and be more careful. for instance I just bought a 95 525 and a DD and I don't want to put money in it so I bought a nice 1 owner low-mileage one that I won't have to do the timing chain or clutch on or anything else for a while.
Hope that helps.
"It is needless to say that self-propelling vehicles, like other machines, will never do as much for one who does not understand them as for one who does."