If you want tight, punchy bass, I would suggest one or two 8" subwoofers, in a sealed enclosure.
As you already have a box, you need to match the sub to the enclosure.
The issue is the .5 cu. ft. per sub...... a bit on the small side for mainstream 10 inch subwoofer. You can stuff the box, which will make the driver "see" a larger enclosure, but it only works up to a certain point. I've never been a big fan of stuffing boxes, unless its to reduce standing waves in a parallel baffle (square) box. You can put a sub with a larger volume requirement into a smaller box, but performance will suffer. Also you have to take into account the woofers displacement(the volume the driver takes up), usually .03 to .05 for an average 10" driver, which reduces that .50 cubic Ft to .47-.45. That size enclosure is perfect(and too large for a few) for most 8's IMHO, but too small for your average budget friendly 10" sub.
If the order was just placed, and you're open to 8" subs, that's the route I'd go. JL's 8W1v3 would be a nice fit with some blocking inside the enclosure to get the right volume, or if 10's it must be, then the 10w1v3 would be okay, but the box is about 10% to small..... but that shouldn't affect it too much, just might get dicey at higher power levels. Both have acceptable mounting depths......
10W1v3-2 - Car Audio - Subwoofer Drivers - W1v3 - JL Audio
8W1v3-4 - Car Audio - Subwoofer Drivers - W1v3 - JL Audio
The 8" model is a 4 ohm, the 10" is a two ohm.
You don't need a capacitor. Won't hurt, but you don't need one.
I would look at five channel amplifiers, most today have built in x-overs, subsonic filtering for the low channel, and can easily drive a system like you're after. Plus ease of install....
5-channel Amplifiers at Crutchfield.com
As far as interfaces, I honestly have no clue. I've never used one, always ditched the stock HU...... but never had a spiffy factory HU either lol.