I've got an E90 M3 as well and just picked up an x28i as the family car. The ride is a little stiffer than I wanted in a family car but the handling is great for a crossover. The lack of space may be an issue depending on your needs though. I would recommend getting the 28i over the 35i for several reasons. The 28i comes with an 8 speed transmission (6 speed in the 35i), hydraulic power steering (electric in the 35i), and better fuel economy which will help offset the money you spend fueling the M3
I think the four cylinder engine has plenty of pickup although there is a noticeable lag when accelerating from a stop.