Your cousin's a lucky chap. The Elise is an absolute peach
In terms of wheels I'd suggest you look for period photo's of racing 3 series BM's. They're liable to be fat BBS cross-spokes, 15" would be fine.
I've never heard of Proton for sound systems, and the name doesn't augur well. Again, if I were you I'd be looking for something in keeping with the age of the car. An '80's Blaupunkt would be just the job. Otherwise, look at Kenwood, Alpine, and especially Nakamichi.
Bonne chance, and post those pics when you have a mo'.