Hi mate have a look here
http://www.bcae1.com/spboxnew2.htm.
Stick with 12's they will serve you well. As for which way the subs are mounted depends on the size of your boot. As you'll have to take into account that a sub mounted upside down will have to move all the air in the boot.
For the front and back I'd be looking at putting a set of components in both front and back. Your gonna need at least 2 seperate amps. You want to be looking at Class D Mono amps for the 2 subs and maybe a 4 channel amp for the other speakers. Or two separate 2 channel amps. Hope this helps buddy.
Look towards Audiobahn, Alpine, Pioneer, JL audio, JBL.
Infact have a look here
www.bassjunkies.com