Better do a search on VANOS. I believe that car has it, and the system does fail. I would check it out before buying not just assume it is harmless valve noise.
It should NOT make noise. Do you think it did when new? Not a chance. My notoriously noisy M30 runs like a sewing machine, a German one. The M50 should be quieter yet.
Automatic trans? Better watch that with high miles too. These cars can be money pits in a big hurry. All you need is a cool head, a good inspection and test drive. If you are not comfortable with this, then maybe the BMW is going to blow your budget?