i'd go with at least one 10 or two 8's... my room mate had an HX2 8 in a ported box in his WRX and amazingly enough, it got as loud as an old set of Lightning Audio 12's in a sealed box. I'd settle for nothing less than a single 12 to be honest. You can always dumb down your amp or head unit settings to match the level with your ears' preference.
Any spending preference? I can get Orion stuff at good prices and will swear by their good sound reproduction and quality of build
