hummer h2 ?15's

I have a mtx 1000w rms 2ohm stable 12.6v amp

I found 2 15" fosgate p3's on ebay for $300

are they good speakers, if not what do you suggest?

and how much sq ft should a sealed box be for each speaker

I also would wish to keep my stock stereo, how would I be able to convert to clean rca outputs for aftermarket amp?

www.audiocontrol.com for a really good LOC with sub level control will take care of the interface between factory radio and aftermarket amp. Look for the 6XS model. As far as the subs go, why do you anticipate needing 15's?

Those P Series arent very good... You can find much better 15s for $300.