I would definately say you might be better off looking into a mic upgrade 1st. You can pick-up the AKG 1000's for a relatively cheap price, plus you can run them off of a 9V. You may also want to look into finding a used pair of Blue Line (AKG 390 series).
The most important component to build upon are your microphones. As far as the minidisc thing goes, if you don't mind changing you discs more often,stick with it for now. DAT will soon becoming more and more obsolete and you will find good deals on decks later on down the road.
IMHO, once you settle on a pair of mics you like, build from there.
Dan