Your amp really depends on if you plan on upgrading in the future.
That sub package your looking at is a dual 2 ohm you can either run it at 1 ohm with a single sub or 4 ohms, so if you plan on keeping one sub and not upgrading to 2 speakers find an amp that does 600-900 rms at 1 ohm or 4 ohm ( I wouldnt do more unless you really know how your tuning your gains)
That box is bad build your own box.
If your running more rms to a sub then its rated for its important to break that sub in with low gains and volume for a good few weeks so that the suspension loosens up.
What i would do, is just buy a p3 the sub alone the Dual 4 ohm coil(Not the one your looking at), then run it at 2 ohms on this.
http://www.sonicelectronix.com/item_29851_Hifonics-Brutus-BRZ1200.1D-Amplifier-Kit.html And if you want to upgrade in the future just buy another dual 4 ohm and wire it down to 1 ohm. WALA
That 0 gauge wiring kit will be nice too.
Heres a diagram for a dual 4 to 2 ohms
http://images.the12volt.com/12voltimages/1_4ohm_dvc_2ohm.gif
I'd muchhhhh rather not have to build my own box :/ isnt that loaded one perfect for the p3 sub?