I'm looking to spend <$100 at this point, until I get my feet a little bit wetter and figure out what I really want/need.
Maybe too late, but I'm curious if you found a solution to your problem?
So you wanted to go: phantom powered mics >

> minidisc (legacy model)
As mentioned, the UA-5 goes for more than $100, otherwise that would have been your best bet.
Here are your options with usual street price (for units without pre, go mic-in on your MD)
Samson S-Phantom (AKA OSP PPS-2) - $37
Art Phantom II (AKA Omnitronic LH-051, AKA Australian Monitor PPA2) - $47
Nady DMP-2 (AKA Hitec Audio PreMobile 2) - $49
Rolls PB224 - $72
PS-2 - check the YS for any good deals