There is nothing your old piano would like better than to be heard
again.  Installing a new solenoid player system would be a great
idea if you would enjoy listening to it.  Pianos are about the worst
investment you can make today so don't even think of it as an
investment.

If the piano is very rare and has high value, you may not want to
modify it.  There are very few pianos that fall into this category.
Pianos, especially grands, are high maintenance items and have little
value when neglected because it costs so much to restore them.  This
is especially true today with the failing piano market.

So first check out the value of the piano.  If it is an historical
piano of great value, you should probably not modify it but save it
for a museum.  Otherwise add the solenoid player and enjoy it.

If the piano is not in very good condition, adding a player will be
a waste of money.  Therefore it must be restored, if necessary, in
addition to adding the player.  A reputable installer will not install
a system in a poor piano as it will reflect badly on their reputation.

If you decide to install a solenoid system, I highly recommend the
Stahnke LX system, which I believe to be superior to the other "third
party" solenoid piano systems.  The LX costs only a bit more money
than those systems, but I believe the price difference to be well worth
it.

If you want the piano to play softly in a small room, you are better
off installing a stereo music system.  Piano music should be heard the
way it was performed or you are missing the benefit of a live piano.
If you want quiet background music in a small room a piano is not the
right instrument.

The value of the piano before and after will vary depending on the
quality of the piano and its restoration, as well as the condition
of the used piano market which is terrible right now.  A poor player
installation could actually reduce the value especially if it was a
valuable piano to begin with.  A superb installation will only increase
the value but this may not be recoverable.

Unless you are in the business of restoring piano and have access to
wholesale prices and a source of labor you will not make any money
restoring or improving a piano.  Even the professionals are having
a hard time right now.

Best regards,
Spencer Chase
Garberville, California