I can tell you from personal experience that MP3 playing is possible on a 68030 at 50mhz. You have to reduce the sound quality, the maximum frequency, increase the sound buffer and force the songs to be played at mono, not stereo. My prefered player is SongPlayer.
Web browsing is very limited as you already know, but that applies to all Amigas, advanced or not. On an AGA Amiga browsing at 640x512 with some overscan and 128 colors is reasonable as long as you don't forget to install FBlit and FText or SystemPatch at your startup-sequence. Not in great speed, though.
