To be fair I wouldn't expect any major issues on what was effectively a brand new car. It's once they get older that reliability and running costs become a concern - between 10k and 40k miles I'd imagine all he ever had to pay for was 2 services, a brake fluid change and some tyres.
Which, for me, is the problem with an M5. The M5 is probably the car I've always liked the most - but the type of car it is makes it, for me at least, a fairly useless purchase as they get cheaper. I don't think the M5 is a weekend car. It's a do everything car - if you're only using it occasionally and for low mileage then arguably its talents are lost on you and you'd probably find something like an M3 or even an M2 to be more suitable. If you are using it in such a way as to really benefit from what makes it so good, you're going to paying a significant amount of money in order to do so.
I've never had an M5 because every time an M5 becomes affordable, it's also got to the point where it's starting to need big money spent on it and it becoming less and less suitable for use as an everything car unless you've got so much money you don't know what to do with it, in which case you're probably already driving a newer M5 anyway.
The M5 is amazing because 90% of the time it's an ordinary 5 Series with all of the benefits of usability that brings, but 10% of the time it's really something very special. But just how much money do you want to pay to run a car that, 90% of the time, is an ordinary 5 Series, to get that 10%? And if 80% of your driving is for fun, then the fact it'll sit on the Motorway for 300 miles in complete comfort is really not that useful for you.