You better test the transfer speed to and from the SD card first to make sure it can handle your needs.
Also sending MIDI is not so difficult, but recieving is much harder because you need to take the MIDI protocols "runnig status" into considertation, and keeping up wih a steady incoming stream of MIDI messages while at the same time constantly writing to the SD card because your buffers will be small.
This is not an easy project. I would go for the MIDI player part first. Make it work as required, and then in version 2 start to mess with the recorder stuff which will be a serious challange to implement.