I'm not an Apple guy, so can't help with many of your questions but -
Your mini DV deck almost certainly will NOT do HD. It would not only have to be able to read the tapes, but it would require HD outputs.
Your software probably does do HD, since most all do nowadays, but you can check Apple's FCP website to make sure. That will also list the computer requirements.
Blu-Ray or HD DVD? That topic won't be decided for some time yet. If you're serious about HD production, you'll probably need both.
As for whether or not your camera will shoot SD and HD - that's up to the camera. My Sony FX1 will shoot either, and if I shoot HD, it still gives the option of outputting SD. Again, check the manufacturer's website for specific details on your camera model.
We're all in the same boat, though on varying timelines, in that we're all going to have to switch to HD sooner or later, if we haven't already.