I'm not as familiar with pre-1997 5 series but I would look for the usual things as on a higher mileage car. Belts/hoses has the timing chain been replaced and just general evidence of maintenance. Because BMW's have the "maintenance light' that comes on when it is time for oil changes or regular maintenance, and most owners don't have the reset tool to shut off the light, they usually are well maintained as the owners need to take it in for service to get the service light reset. for the do-it-yourselfer, the reset tool can be purchased from most any BMW parts supplier such as Bavarian Auto.
The 3.5's I have ridden in have had excellent power and BMW reliability is usually excellent
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1999 528i 5 Speed, Sport Package, Confortti Software, Racing Dynamics Front Lip Spoiler and Rear Apron, Supersprint Muffler, Angel Eyes Upgrade.