sills,rear wheel arches and boot lid are common areas for rust (the arches starts from the inside and works outwards)

high mileage cars could need a new clutch (given that some peoples driving style in these cars can be less than desired )

thrust bearings (some people recommend upgrading to the X5 ones as there are a little stronger)

viscous fan clutches is a bit of a weak spot,not just with the m5 but a lot of the cars from that era also the resistor for the fan can be troublesome but that's also common on the normal 5 series of the time aswell

normal wear items like engine oil,power steering and diff seals should always be checked but they weren't a problem with these cars but any high mileage car should be looked at for these

personal one for me is the seats (given the age of these cars now they can suffer from wear and and if your buying one with the aim of freshening them up that can be costly )

all in all a great car,lovely car to drive weather its for a spin down the country or blasting through the sally gap
and if looked after and maintained its not going to give you any major headaches

like any high performance car you should do your research on whatever one you go see,bmw can give you any bits of info on the car if its got a service history on it
weather the price goes up or down is dependant on the market for them but they are getting fewer and further between over here