HI there, firstly welcome to MINI 2
I opted for a MINI One because of the cheaper insurance and the availability of ECU mods to compete and even beat the Cooper Performance
As for the wheel fitments you can go from 16" to 19", Personally I think 17" or 18" is about right for the MINI
There is a wide variety of places to get parts and accessories, if you check out the sponsor pages on MINI2 there a various links to different company's specializing in the "New" Mini where you will be able to get lowering springs and various different body parts
The Cooper S does have the scoop your best bet to get one is either on Ebay which I think there is one on at the Min or a local breakers yard