Buy the best one you can find - loads of rubbish about
Tyres are conti's - £170 each if you shop around - M3 specific
SMG box is prone to actuator failure - £1500 repair
"Rear subframe issue" is actually an issue with the bodyshell - they rip out their own boot floors on the welds (spot welds run along the seams and are weak). The issue is caused by a poor design of suspension bush and torque can twist the shell. If it's gone then it's a full axle off job and will cost £££.
Buy a mint low mile car and you won't lose much, it may even appreciate - but these cars are becoming scarce.
A friend has a completely original, mint 20k mile example and he looked at about a dozen cars before he found this one. If you're looking at the lower end of the market then be prepared for big repair bills; clutches, brakes, etc are expensive. As I said - lots of iffy ones about.