Actually avoid any e90 model. Especially 2006-2011 models. I had a 2007 e90 and I can tell you everything that can go wrong went wrong with it, including things like differential and ecu. After the warranty got over I spent around 3-4K every year for upkeep. My recommendation is that you first search for a competent mechanic, someone who's good at fixing bimmmers and doesn't charge you arm and a leg. After that, you can decide which model to buy. BTW, regardless of the sufferings with my e90, I used to love the driving dynamics of that car. Totally awesome, so awesome, that I ended up with an M3.
Edit: Added this consumer reports link as well:
http://www.consumerreports.org/used-...-avoid-buying/