Advice on buying a BMW 3 series

Have been looking to purchase a first vehicle. I'm currently a student who works two jobs though they are both pretty seasonal jobs in terms of how much work is available (one there is no work in the summer). I still need to save up a little more before I feel comfortable buying a car (but we are heading into winter). I've been looking at either a e90 (maybe e46) 3 series from the 2006-2008 age for the 10,000-17,500 price range (AUD). Though peers think I'm insane as sometimes repairs can be too much on BMWs.
What is a reasonable amount to spend and have saved up in order to purchase a 3 series and be able to afford repairs as they fall due.