Good find and welcome.
I purchased my M50 powered 525im with a bit less than 400,000km. Original everything including ZF tranny. While its different than the 535 manual gearbox, its still running flawlessly in every respect with more than 500,000 km and counting. I did just finish an engine rebuild due to low comp on one cylinder, but to the layman, it drove just fine before the work.
I'd say put the mileage in the back of your head, and inspect the vehicle solely at face value. Someone here should chime in on the Getrag? gearbox.
As for other things to watch, its a long list so try the archives or go to Bruno's "what will break page"
Good Luck,
Dave M