I have an 87 325 that has over 230,000 miles on it, and it's still got the complete original drivetrain. It drives great, and it doesn't use or leak a drop of oil. I do know that the car has been meticulously maintained and at the dealer for most of its life. It has all BMW parts on it - no aftermarket stuff. You'd never guess it has as many miles on it as it does by the way it drives and looks. Just be sure that it has been well-maintained and it should be a reliable car for you.
I've had mine since the end of May, and the only things I've done to it are replace the fan clutch and thermostat (easy and relatively inexpensive job) and replace a broken odometer gear (it broke about a week after I bought it) - also an easy job.
Definitely get the service records, especially to prove when the last time the timing belt was replaced.