... ehm, this can get a loooong discussion... 😉
a) iMovie is concepted for miniDV, firewire connected... any other thing is ... "possible", but workaround, huzzle, trouble, problems... and: firewire is also a standarized protocol: any fw connected camera can remotely controlled by iM etc... = convenience
b) miniDV, precisely: dv, is a recording and editing format; compare: 1h dv = ~13GB, 1h dvd = 2GB... ok, compression techniques gets advanced ... but, as said, the videoDVD, precisely mpeg2 was concepted for delivery, not editing... (talking of advanced: mpeg2 is older then dv...)
I wouldn't be sure, that another camera/device can "squeeze out" better quality from your old files... you've recorded them in mp2, done. as said, try the options I've listed, decide on your own..
investment: I would invest into a good 3ccd miniDV camcorder; if you really want to spend some money, get one of these HD beasts... ok, no disk delivery until now (BlueRay/HDDVD?), you have to store final projects on tape... and you need a really fast Mac for handling HiDef content.. (besides: the Sony HDcamcorders use a kind of mpeg2 tp press all data on the small tapes...)
summary:
* test software solutions
* buy a 3ccd miniDV as "inbetween" for the next 3 - 5years (you can use it as convetrer for your old stuff) - then HiDef is standard, harddrive/flashmem-corders are affordable...