The E90 series is well proven, before i bought my E91 330i M sport I found the common failures were
ABS pump (well the electronics) but this tends to be on lesser used car and this happened to mine
Climate control sampling fan can start to squeak which can be annoying,
Broken springs that seem to effect all BMW's for some reason
Suspension bushes when you get north of 120K but that is to be expected
The tourings can have issues with the central locking and radio reception, all the gubbins are in the rear spoiler and the seals fail
Make sure it is has had regular oil changes the cam chain tensioners can fail on neglected cars
Don't buy one with poverty spec there are loads out there to choose from get some toys, just a word of advice if you don't like a hard ride then avoid the Msport it looks the part and handles great but it can get a bit wearing.
But like the other guys I have had mine for over 4 years now and apart from the ABS pump which happened soon after i bought the car so was covered by warranty it has cost me nothing beyond servicing and a set of rear tyres