The 6 Series is now selling from around $5,000 (or less) for a decent car to around $20,000 for a very low-mileage, super-clean car (including the M6). This is a fraction of the equivalent cost of a new one in today's money. Compare to 1986 prices, which ranged from $47,000 to $65,000.
Many feel they are too exotic and costly to own and maintain, but they can be as reliable as the 5 Series upon which they were based. In most cases, too, the 6ers have been better cared for and maintained than the average 14- to 27-year-old car.
The majority of these cars have over 100,000 miles on them, many considerably more. They require a BMW enthusiast who can do some of the work themselves or an avid lover with a healthy checkbook and a good mechanic at hand. The 6ers are still young enough that parts are readily available, yet they're also old enough to have achieved near-classic status.
There are two groups of people from which I'd recommend buying one of these cars: original owners and BMW enthusiast owners. Those who have had the car since new have probably maintained it well; having kept it this long usually means it's a loved one. This owner will also probably have all the service and repair records as well as complete knowledge of the car's quirks or foibles.
Cars from this first group fall into two categories, stock and upgraded. The stock car has likely been used only occasionally, may have the TRX wheels still mounted, and is usually an automatic model, including the L6. Upgraded cars from this group will probably have had the TRXs traded out and may contain a chip upgrade, Bilstein shocks or a short-shift kit (most of these cars are manual five-speeds).
From a BMW enthusiast owner, expect good maintenance records and a number of upgrades, all of which are well documented. If you don't want to mess with modifying the car yourself, these are the ones to search out.
The choices, then, are simple: the stock car with super low miles (some can still be found with 20,000 to 40,000 miles on the odo); or the well-used but also well-maintained examples with desirable upgrades. Anything else means lots of time and money ahead.
Certain upgrades are now considered mandatory. First is replacing the TRX wheel and tire package. Some experts recommend deducting money if the car still has the TRXs; the only reason to keep them is if a collector wants to show a totally original car.
Above all, do your homework. Go to on line discussion groups, check the classifieds on-line and in print, and always check out eBay. Look at and drive several cars just to get the feel if you haven't previously owned one. These cars have a large group of avid followers, and there are many good sources of information, including, of course, my 6 Series web site, www.bigcoupe.com.