Heya Kamaro since you are spending 500-600 bucks I would go with one good 12" sub and a good 400-500watt amp RMS. You will need to put this sub in a large ported box and it will be real loud!!!! Average size for one 12" sub is 2 1/4 cubic feet. Also make sure to build the port out of wood rather than a whistle port . Brands I would recommend Kicker, Orion, MTX and JL. Amps the same as the sub brands.
Hope this helps and keep us posted how that stereo is going!!! Do you have an alarm??? Sounds like maybe you should get one to if you install this system!!!
See Ya,
Karen

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Karen Wells, Moderator
The Mustang Works Magazine
http://www.mustangworks.com
1969 Mustang (Project Rice Eater)
1999 Silver Mustang Saleen S281 (SQ car for stereo contests)
SQ Goddess
My Stereo Bro "tireburner163"