No, you can get Alpina, Schnitzer & Hartge bits for these cars. But they are even more expensive than the OEM parts (of course). Plenty of parts for LHD cars.
As far as costs go, I think it would be comparable to running my car. It is one of 70 built and has the full M5 suspension package (EDC shocks are NOT cheap to replace!) It is awesome though. The torque in the M60 makes it hard to launch without toasting the clutch, the dual-mass flywheel and hydraulics leave next to no pedal feel.
Dial in 3000 rpm and drop the clutch is the best way to launch my car. You can only do about 3 starts (in close succession) before it begins to heat up and slip. However, doing the drags, it gets enough rest. On the clutch front, don't lend it to someone who isn't going to look after it. The clutch is fine for the torque of the V8, assuming you don't ride it.
I'm not sure about your 540, but with the ASC+T fitted, I can (and do) loan my car to anyone to drive, without being concerned about them wrapping it around a pole (if they manage, they deserve it). An M5 is not such a forgiving car.
Anytime, I have a service and it costs me less than a Grand, I'm happy. An well looked after M5 will hold it's value.