If your minidisc has a line out you should use that. My Sony MZR30 has, most newer ones don't and you have to use the headphone output.
MP3 would be hugely preferable to WAV - 1 minute of music is 10Mbytes of stereo 16 bit WAV, only 1Mbyte as 128k MP3 (approximate figures). Unless you're both on broadband you'd be waiting serious lengths of time to upload and download it all.
Snail mailing a MD is a really good way of getting a lot of good quality audio from A to B!