Welcome to the board
. Plenty of good advice so far but you are welcome to my opinions too......
Regarding MPG; I have averaged about 22mpg in the 100k + miles I've had the car. I do a lot of long distance driving though which is good - fortunately I rarely have to sit in rush hour city traffic. I once reset the computer before caning along my favorite B road and got about 7MPG
. I use exclusively Shell V-power or Tesco 99; both are great.
As far as buying an M5, there seems to be a huge disparity between the good ones and the bad ones and it does not necessarily just depend on mileage; the more you can find out about the history of the car and how it has been treated, the better. With the M5 you are better off finding the best example of the car in roughly your choice of spec, irrespective of purchase price.
Maintenance is not bad for a 400bhp V8 supersaloon in my opinion. I have kept mine up well and it has cost me what I consider to be a very reasonable amount of money over the years for what it is. Don't be tempted to skimp on it though - if mine has ever needed something doing, it has been done straight away with proper parts by someone who knows what they are doing. It's good to remember that this was a £58,000 car + options when it was new.
These cars are built to be used; if you get one don't abuse it, but for Heavens sake don't baby it either. I drive mine quickly but with a degree of mechanical sympathy and it has responded very well indeed.
Brakes - for normal road use they are stunning. For fast road use they are more than up to the job. For track work you're gonna want to upgrade them. Full retardation should feel quite simply awesome but on a track the car will get to such a high speed and weighs so much that the brakes will get cooked in a lap or two. There are good upgrade options if you are a trackday warrior though.