Reddit reviews NVX 100% Copper 4-Gauge Car Amp Install Kit w/ 2-Channel RCA, Up to 1000 Watts RMS [XKIT42]

The first link would be the better option. That Planet Audio amp is super cheap in more ways than just price. You'll be better off with the MTX system. Add another $50ish for a wiring kit and you will be set. When shopping for wiring you will want 100% copper (referred to as OFC; Oxygen Free Copper). What you typically see in cheaper kits is CCA (Copper Clad Aluminum) which is cheaper to produce but will not be as conductive as OFC.

This would be a good option.

You will not have any issues with getting low bass out of those subs. A different kind of enclosure will achieve different results. A sealed box like you have linked will be very punchy and tight. If you want something with more boom and perceived loudness you will need to be looking at a ported/vented enclosure.

I would use the KnuKonceptz OFC kit or the NVX kit. I like the fuse block on the NVX kit as it is pretty beefy.