hi,

I am building an mp3 player with the following devices: IDE drive, pcmcia card, LCD & button.

Well, so why not to use AT89C51SND1? It has most you need: IDE/ATAPI, USB, Compact Flash, SPI, I²C, PCM/I²S output etc interfaces.

How many ports/pins is this going to require?

I think this is most last question you may ask yourself when start to do such project. When you have estimated your task completely (ideology of the project, block schematic, algoritms, your software and hardware knowladge etc) then you come to select derivative (AT89C51SND1, another [fast] 8051, simple 8051 with external mp3 decoder, ARM, PC etc). And then you just select appropriate package from available ones.

Regards,
Oleg