Yeah it would be good to see your results!
15mm on the rear should fit fine but whether or not they'll give you enough of the look you desire, only you can decide.
12mm on the front is probably pushing the limits of what will fit but will give you a great look! I've got 15mm on the fronts on mine (ordered 12mm but got sent 15mm in error) - that's as big as I'd go in terms of look and physical fit. I don't think you'd get any bigger on the front, and any wider would give a disproportionate look when compared to the rear. In hindsight I wish I'd gone for 10mm. It would improve the look of the fronts but also make the rear seem
Personally, I'd always pay the extra to go hubcentric. It'll save any additional undue stress on the wheelbolts and ensure that the spacers are aligned properly to prevent any kind of wheel vibration at speed. Each to their own though.
Look forward to seeing your pics.
2006 E90 325i SE - Silver - Manual - Xenon Headlights - Full Leather Interior - Extended Interior Light Package - DCC - 18" Black Multispoke Alloys ~ Style 269.