1) if you want a cheaper solution, get Mac audio. If you are willing to spend more, like twice, get MB Quartz (incredible). but then you'd have to sound proof your doors for the best results.
2) tweeters: major improvement on the sound quality. The speakers I mentioned before are in most cases component speakers (the ones I recommend). This means you have the woofer (along with a crossover) and the tweeter (of course with its crossover too). As for the tweeters, try and place then as precesi as possible to your ears' level.
3) You could make one out of MDF, and cover it with some vinyll or something...
4) for an amp, I always recommend Hifonics, but they are a bit pricey. You could go with an Alpine or Magnat amp.
5) don't worry, theese amps don;t go very high on volume, but they do reproduce a clear sound.
5.a I've seen enclosures smaller than what the manufacturer recommende, and they sounded pretty good. But, I'd say, stick to the calculations in terms of building an enclosure. You could mount it in the spare tire well, or on one of the sides of the trunk. As for the diameter of the sub, it actually depens on what type of music you're listening to.
5.b do you mean, free air subs?
5.c You could mount the sub on the parcel, but I can't guarantee you'd get the best results, especially if it's a 3 door car (too much air loosing around), so not quite recommended.