Cabrio, so you have the lower arches. Air ride?! Never seen that on an E30, but okalie dokalie.
For a 7.5" wide wheel running 225/35/18, you'll want an offset in the range of ET23-ET27. The closer to ET25 the better.
For a 7.5" wide wheel running 215/35/18, you'll want an offset in the range of ET20-ET30. The closer to ET20 the better.
Try to get it in the ET23-27 range so you can run the 225/35/18 tire size. Juding from the air ride you don't push the car hard so the extra grip wouldn't really matter, but hey, it'll look better.