The 850 was a 2-door coupe and of rather limited production, I wouldn't be surprised if had less numbers than for the VX. And, even for a 12 year-old car, it is still looking fairly sexy. But, as Spike pointed out, if the 12-cylinder needs any repairs, the cost will be ass-kicking-tro-nomical, and the 12-cylinders are not known for reliability either.

Plus, you might want to look at safety systems. Did even BMWs have anti-lock braking and air-bags back then? If you can get specs like stopping distances for the 850, almost certainly they are longer than for the VX which has one of the shortest stopping distance of any vehicle on the road today due to the brake design being borrowed from an SUV that was 500lbs heavier.