Hi Charlie,  There are two retrofit systems available: PianoDisc
and QRS Pianomation.  When properly setup, both systems can function
beautifully.

This first step for you, though, is to determine that the piano itself
is in good enough condition to make adding a player system worthwhile.
The condition of the action, pinblock, etc., must be verified by
a piano technician, otherwise you could be adding an accessory costing
several thousand dollars onto a piano which needs thousands more in
servicing.

With the QRS system, you can play MIDI files from a laptop computer,
through an available MIDI IN/OUT cable.  On their most recent front
ends [control boxes] though, I would suspect you'd find it more
convenient to use a Compact Flash card.  Since MIDI files are so tiny,
you can fit thousands upon thousands of songs on a single Flash card.
If you'd like to hear the synthesized backgrounds on MIDI files, a
sound card can be easily added to the system.  PianoDisc systems also
have MIDI capabilities, though I've not had occasion to utilize them
on their most recent front ends, so I can't give specifics.

QRS and PianoDisc each also have their own propriety music, that you
may want to check out.  Some people base their purchase decisions on
this.

No matter what system you chose, the quality of the piano, and quality
of the installation, will have a tremendous affect on the performance
of the piano.  It would be ideal to hear an example of the installers'
work, to verify the quality of their installations.  Paying particular
attention to how much expression you hear when the music is being
played (some songs though are far more expressive than others) and how
quietly you can turn the piano down, will be a valuable guide for you.
Good luck and have fun.

Herbert Lindahl, RPT
South Windsor, Connecticut
http://www.herblindahlpianotuning.com/