Superb cars in every way, so capable of doing everything you could ever want. Lovely crusier and a hooligan when pushed.

BUT SO SO SO MUCH FUN!!!!!!!!!!!!

Dont get sucked into the cheaper cars, they WILL be nails that need 2k spending on them once you cut through the shit.
You want to go for a facelift model if possible. And as with any BMW you want to get the best spec as you can...Most of the cheap cars DONT come with Sat-Nav, H&K Stereos, Heated seats ect...Thats why they are so cheap.
As people have said, If you get one that is less than 10 years old and has full BMW service history and there is a Boot floor issue, it 99% will be fixed FOC from your BMW dealer as a goodwill gesture. So either get one that has all ready been checked/replaced or 100% Make sure you buy at least an 04 plate car.

Servicing can be dear if you dont look after them. I had an oil service done on mine from BMW and it was £169.00....Dear yes but kept my FULL BMW service history uptodate.
Tyres are expensive....Simple as. But in a car like that who in there right mind would fit cheap tyres. I bought 2 rear Bridgestone RE050's for mine and it was £500

Brakes are now not as dear as people think. I replaced all the discs and pads on my car. Genuine BMW discs were £420 all round. Pads and sensors were £100

As for running them and fuel.... its an M3.............On a 200 mile run in mine sitting at around 80mph i get 31mpg. Blasting along through the B roads it was at 16-18mpg.
If i stuck £50 in it for a weekend blast and a drive about it would do 140mile ish.....