The largest steel wheels you'll find for the MINI will be 16". No one makes a 17 or 18 inch steel wheel in 4x100....I've looked hard.
If you're doing this for hyper-mileage reasons, I would think the weight distribution on the 18's would negate most of the benefits. I could be wrong as my hyper-miling knowledge is limited at best.
Sounds like you're looking for a super fuel efficient wheel and tire combo. Might be better to look at 15's, as you can get some narrow tires on there like 185 section. Not sure what size Prius wheels are, but I think they're around 15's, so you could get some low rolling resistance tires, too. You're gonna be hard pressed to find an 18" wheel less than 7" wide, where plenty of 15's come in 6.5" width.
