I am under the impression that a ported box will sound louder at a given frequency (tuned by the port's size, shape and length), tailing off either side, whereas a sealed box will have a more consistent level across the frequency range. I am delusional ?

You might be better off (sound wise, not necessarily financially) buying a box with subs that have been designed to work as a pair and flogging your two subs.

You could also ask on the following boards:
http://www.caraudioforum.com/vbb3/index.php
http://www.talkaudio.co.uk/
http://www.carstereo.com/forum/index.cfm

I don't go on any of them, just found them in google so can't comment on how good/bad they are.

Godd luck.