Check for:
Dubious history, cheap bodykits and M lookalikes, excessively lowered suspension, general neglect and accident damage

Summary:
With the new 6-Series on the road, the old BMW 6-Series coupes have crossed the boundary from cheap but flash older cars to be seen as proper classics like their predecessors, the sleek 3.0 CS, CSi and CSL ('Batmobile') models. At the moment, they can still be had at very reasonable prices - even the 286bhp M versions, which are serious performance cars though values of the more desirable models are creeping up. Of course, many a 628/633/635 has been tarted up to look like an M, some more successfully than others, so if you're after the real thing, make sure you know your engine and chassis codes.

The earlier 6s came in manual (CSi) or automatic (CSiA) form, the auto gearbox having just three ratios until 1983. From that point, the continuing 628 CSiA and 635 CSi both came with the new four-speed auto box only; the M635 had a close-ratio five-speed manual system. Right from launch in 1976, this range had a sophisticated level of equipment; the first 633 CSi/CSiA had fuel injection, a limited-slip differential, variable-rate power steering and ventilated disc brakes on all four wheels. On the inside, it had full leather upholstery. The 635 CSi, which followed two years later, added uprated suspension, a higher rear axle ratio and front and rear spoilers, and from 1980, it came with the option of the close-ratio manual gearbox. The 628 CSi and CSiA, launched in 1981, had a similar level of kit, but no spoilers (plenty of owners added them anyway) and all models from June '82 had revised suspension, digital ignition and a new fuel-injection system; the 635 CSi got standard ABS and, from 1984, the auto gearbox came with the option of economy and sport settings. The M635 joined the range in early 1985, boasting M-Technic sports suspension and styling, and all models were revised in October 1987, getting a slightly restyled front end and more equipment (including standard air-conditioning); the 635 CSi gained an extra 2bhp, and a year later, self-levelling suspension.