Alpine System?

Depending on how far your amp is away from your battery you can use a 2 gauge or 0 gauge wire, but the 4 gauge is too small especially if your going to be running another amp for the speakers. If you purchase the single 4 ohm subs you can wire it in a 2 ohm load for SPL :

This will be the best configuration for SPL opposed to running the dual 4 ohms subs in a 4 ohm load. The single 4 ohm will make use of most of the power from the 7801 which is 2 ohm stable. Also take into account that MTX amplifiers are rated at least 25 % less than when they come to you. The rated specs say 800 watts but your amp will have at least 1000 when tested at the factory.

If you go with a ported box ( all Sledgehammer enclosures are ported ) it will give you an additional 3 db of SPL. Here's the spec of the Sledgehammer enclosures:

Here's a thread about buying online in comparison to an authorized dealer for equipment.... it's a good read. Should only take 5 minutes but well worth the time. Lots of good information from various member's of the forum and just good to know when you decide where to buy.

That's a very reasonable price for the equipment. A HO alternator you can try a search through Google or on this site.