hi,

My only concern is its speed. At 20 Mhz it will be hard to get decent transfer rates

Rates for what?
First of all, this derivative supports x2 mode. It means that one machine cycle takes not 12 but only 6 oscillator periods. So you may use it with 20MHz oscillator but speed will be like you work with 40MHz one.
IDE/ATAPI interface may work up to 12 clocks per 16-bit access. In theory it gives you about 3Mb/s transfer rate. In practice it will be far less but still enough for most tasks. In fact, mp3 does not require so high transfer rates anyway.

Regards,
Oleg