1) Depends on the amp you'll choose, you'd go for the lowest stable ohms the amp is capable of running at. It'll also depend on how many subs and the wiring configuration you'll choose.
2) The MRV-T407 would be powerful enough for a single efficient subwoofer.
3) The number really doesn't matter. More is not always better. A single 12" can sound incredible and a 10" triple box can sound like poop, or vice versa. I usually go with a single sub for simplicity. For more more volume, you'd just go with a sub that has higher handling and displacement abilities.
4) Yes (provided the amp is capable). And, the frequencies subs play have little or no stereo separation.
5) Depends on what you're after. Sealed will play deeper and (usually) more accurately while taking up less room at the expense of efficiency (you'll need more power). Ported will play louder with less power but can be 2-3x bigger than the sealed box. If not done correctly, they can sound boomy, muddy, and inaccurate.
6) Best/easiest place would be in the trunk against the rear seats facing the rear bumper. Build it with an angled back to fit flush against the rear seats and use some velcro to keep it from sliding around.
7) Depends. As you go bigger the subs will play deeper and be more efficient. But, they'll require larger enclosure sizes. Figure out how much trunk you're willing to sacrifice and choose the number and size of subs from there (also keeping in mind how much amplifier power you've got or will need to power them).
BTW, my brother-in-law has got a pair of 10" Type R subs in his L and really likes them (I thought they were pretty decent too). Although there are plenty of subs that can outperform them in the bump/$ measure, you'd probably be happy with them.
