WR-one

They have 17 x 8; 235/45/17 (and I think a 53 offset) - looking to go for 18" - think 235/40/18 is nearest 18" size to retain overall diameter as near as - am I right ? Also conisdering going for an 8.5" as opposed to an 8" - if the OEM is a 53 offset which I think it is, what should the 18 x 8.5 offset be ? Also is there any material advantage to having an 8.5" wheel over an 8" wheel given the they will both run 235's ? Will there be any problems with fitting AST coilovers with it ?

Lots of questions - cheers!