Even an 040@40MHz doesn't cut it if you want full quality mp3 playback (not half rate). An 060@50MHz can only just achieve it. Don't expect to be able to multitask though! So all in all, I'd stay away from mp3's and stick with wav's or aiff's (which are much better sound quality anyway).
If you're determined to play Mp3 on Amigas at real quality, you need hardware decoding, like a MasPlayer or Delfina. They work well with Amiga Amp.