Hi.
Well, at 185.000 Km most part will either have been worn out and changed or you will have to start to repair the the front and rear chock absorbers, the caster bearings (rubber bushes) that keeps the wheels straight when you break, etc, etc. The altenator may be due as well.
The engine may be due for a major overhaul (re-build) but if has been properly maintained, it can last you another 150.000 KM.
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If you can find an M5-competent BMW Dealer, have him check out the car.