It isn't just the minidisc units that need a pre-amp, I run CoreSound Cards these days with my D-100 DAT deck. I am unable to use the line-in with the mics & battery box, just no way are those mics putting out enough signal for me to go line-in (and I tape loud electric shows). I always end up using the mic-in. A stock internal mic-pre is sufficient, it doesn't need to be some high-end external mic-pre, but the Sony minidisc unit you are talking about has no internal mic pre at all, just a straight line in, which is why I think you will be most unhappy with the results from running mics directly into the line-in.
The line input provides no gain (even if it allows you to set the levels manually), so unless your mics are outputting a line-level signal you will likely get poor results. Some mics run hot enough to successfully run into the line-in, but my CoreSound mics never have. I always need the gain provided by the mic pre-amp.
I bet you could find a used Sharp minidisc unit for a cheap price if you do some hunting around. Ebay can be sketchy but if you use common sense, ask the seller questions before bidding, and use your gut instincts you can usually come out alright. I get a lot of stuff off of Ebay, but I am pretty careful most of the time!