Well 50.000miles aint that much on a car like the M5. It's not a Ferrari. It's made to be used. 120.000mile M5's are in my point just as "fresh" and just as fast as 40K mile ones if looked after. OK, a new car is always a new car. But since the M5 has got for example almost the same HP/litre ratio as a 520i there is no problem.
Don't stare only on the miles, try and find a really well kept car from a responsible owner.