very reliable, my friend has an e30 with 200k+ on the motor, runs like a champ, girlfriends 325 has higher mileage hasnt had any problems. i have 2 e36's that have never really gave me any issues. My dad drives an e36 m3 that has never had one problem and my moms 2000 328ci runs like its straight out of the showroom.
Like every car you can buy lemons. But for the most part you do not have to worry about that, i would take the car to a mechanic before buying the car, if it all checks out get it! are you looking at a 535?