Creamedweasel

Weld on some larger brackets on the inside of the door :P Sure it would stick in a bit, but who cares? Otherwise I would go with your first idea, it wouldn't sound too hot though, so I wouldn't spend more than $200 on your comps.

Dukk

A decent set of 4" components and the 8s may work quite well. I would explore this option. You could always add a set of 6" midbass later if you felt it needed it.