if you want a coaxial, i would make a plate for a 6.5". then i would look into the pioneer tsd-170r rev series (yellow cones). it is actually an oversided 6.5" but its cool. that is one of the best coaxials i have come across. the zapco i60's (i think) are a good speaker too in a coaxial. they sound smoother, not as harsh as the yellow cones, but not nearly as loud without disorting.
in a component set, that is a whole different ball game. i would look at the alpine type r's. focal 156h's. or maybe the polk mm6 momo's if you can find them cheap.