Since my e90 didn't have fold down seats or a pass through I found some at a junkyard for $350. I also had to order the rear seat latch releases and the top trim piece. Make sure if you go this route you get the top and bottom seat, as the top fold down won't work with the other bottom. The seat belt buckles latches are also longer on yours vs the fold down seat but no real need to switch them out. Lastly the middle seat belt disconnects on the fold down versions so you could or not get that too. I still have the orginal seat as it's napa leather and one I bought is leatherette. You can't tell at all unless you feel a front seat and than the rear, but none-the-less the jackass at the junk yard sold it to me as leather.
Good luck if you cut the current one, I think that will work fine and won't be that hard to do. Post up some pictures.