Yeah, but when you go down the road of DCT or manual swapping it you're in a whole new world of money. Now you're talking about a R1m (maybe even more) project to have it all sorted. At that price you have your pick of M cars including stuff that might have serious legs in terms of appreciation (looking at S54 engined Z3Mcoupes for instance).
I would not rate it any different to a e9x m3 in terms of maintenance.
Critical areas, bearings, vanos oil pump, main oil pump and injectors are shared by both.
SMG is one big difference. I seem to like it but would say its a matter of taste. To me its exotic and a different driving experience than any other. Mostly driven in manual mode anyways. Repair wise with some diy the motor can be repaired or replaced with some aftermarket solutions.
Other stuff I did was cosmetic and voluntary, bushes, mounts etc is due on most performance bmw's by the time they reach 100km.
Regarding cost I think its fair to say to budget 100-200k depending on how restored you want the vehicle and how much diy. I would easily rate mine as the best condition most reliable out there at the moment.
Decent condition and low mileage e60 M5s are being advertised for 700-900k, probably not selling there but buying a decent M6 for 300k,adding 100k leaves you with a very enjoyable and reliable car with limited other options at that price. Anything else you buy for 400-500k is probably also going to need some work down the line.