All other things being equal, and assuming proper maintenance, your 635 will be more reliable in everyday use than any E9, despite what other posters are bound to say.

HOWEVER, since your car is a garage queen that is driven rarely, and presumably in good weather, an E9 may be for you. As stated by others all E9s have rust somewhere, or will as soon as you drive them any distance. They are mechanically simple, however (avoid the D-jet injected ones if you want simplicity) and can be repaired/maintained by everyday folks. If you buy one, buy a warm-climate, low rust or no rust car that is in the best condition you can afford. Coupes are very expensive to restore properly, and are almost never resellable for anything close to the restoration cost (excepting CSLs, etc.) You're better off to buy one that is nice to begin with, and just enjoy it. Good Luck!
Colin

I intentionally sought and found a D-jetronic injected Coupe because in reality, fuel injection is simpler and easier to maintain than nearly any carb set up that uses more than one carb, and many single carbureted engines as well.

This board is replete with horror stories of those that still struggle with the Zenith carbs, which apparently require something of a guru to put into tune and into synch with each other. Although the Weber conversions are available and you can at least get parts for the carbs, they are expensive to buy and initial tuning is almost solely in the province of the professional mechanic due to the Webers extensive range of venturi sizes and other parts. I can tune my D-jetronic with a voltmeter, and a couple of screwdrivers, and if I really want to get sophisticated, I'll go to a smog station and tune it with their tail pipe sniffer, you adjust it to a ppm of the CO output with a screwdriver, it's almost too simple.

Now, there are those that will tell you that the "brain", actually the electronic control box, is expensive and it is; going for about $1000.00 give or take a few bucks, but they seldom fail. Bosch had been producing "brain" boxes for the lowly fuel injected Volkswagen Squareback & Fastback for years before BMW decided to adopt electronic fuel injection for their flagship Coupe. VW had failures in the early days, the late sixties, but after those initial problems were solved, the electronics were the least problematic components on the cars that had fuel injection.

The D-jetronic cars are much more powerful than the carb models. They have 9.5 to one compression ratio which generates more torque, allowing a 3.25 to one differential gearing, and more ultimate power was provided by a combination of compression ratio and the ram induction fuel injection manifold giving 200 DIN horsepower instead of the 170 DIN horsepower of the US models which had 3.45 gearing.

I'd recommend getting a fuel injection model over the carb model in all cases except one. That single case is when you intend to pull the carb engine out and install a late model 3.5 liter Motronic engine which is the real "Cadillac" of the fuel injected engines by BMW, giving completely programmable fuel maps and ignition.

Pat Hines