ohm choice?

Don't expect that Power Acoustik amp to actually produce that kind of power unless you don't mind 50% thd.  If you use that amp I suggest not loading it with 1-ohm as they do tend to smoke... what's the power rating on your subwoofer?

IMO a capacitor is not needed. The time and money is much better spent with a "big three" ungrade (you can read up on that here if you already haven't..it's the only way to properly deal with high powered systems in a vehicle).

If after that there are any issues it is time for a high output alternator...a capacitor will not make up for wires and a charging system that can not handle the draw. If you really want to use a cap do that after everything else is as it should be or better..then it is icing on the cake so to speak...and can look cool, but it won't solve or prevent problems..trust me.