Soldato
I agree with pretty much everything Fox has said here. Having run a 540i for a number of years in the past, I know that the running costs of these cars can be astronomical if you buy a bad one.
My advice is the same as it always is. Buy the very best one you can afford but leave some money in the pot for any unforseen issues.
Many M5s are (sadly) heading in to the "banger" territory having been owned by scrotes who can't afford to service them. And there's a saying that goes with performance bangers: "It may seem like supercar performance for banger money, but it comes with all the banger problems for supercar money"
Indeed, you have hit the jackpot if you find a 1 owner from new car, or a car that has had 2 owners, the last one who had bought it when it was worth a good £35-40k or so.