I'll answer a couple questions, hopefully other members can answer more.
Those particular wheels (DS1 aka Style 22) I don't believe were offered as staggered. 7.5 inches is indeed the width for front and back. 8.5 inch wheels should fit fine if you choose to go that route, as long as the offset is near stock. The centercaps will be available from pretty much any online parts store or dealer.
89 octane is the minimum recommended by BMW for these cars, and I've been using exactly that without issue. I've tested higher octane and have found no advantages.
Beautiful ti by the way, hope you have lots of fun!