My experience comes from an Alexander & Pils made in France about 1860.
It is a very complex action and has many adjustments.  If it has the
felt hammers (percussion-flute) that hit the reeds, expect quite a bit
of time lying under the heavy action getting the escapement adjusted.
What contrast to the beautifully simple and effective design of the
American reed organ.  My harmonium suffers from warped keys making it
playable in dry weather.  I'm presently seeking info on permanent fixes
for key warpage.

The positive pressure bellows and reservoir requires lots of time if the
leather seams are gone.  There is an 'accordion' fan-fold in the large
reservoir (3 leather seams).  The bellows are wedge shaped however and
would be slightly easier to rebuild.  After 130 years, the leather was
still good and required only minor repair.  The quality of materials was
superb.  Expect a few broken reeds that may be difficult to replace --
each is screwed in at the bottom of a 3-inch box.

One of the most difficult things would be the seal between the windchest
and its base.  Since the windchest has separate compartments for each
set of reeds there is a mating surface for each compartment.  Expect
some leakage between compartments.  Tho note critical because there is
a open-to-atmosphere valve on each compartment when the reed bank is NOT
operating.  This seal is a leather tube or welt filled with what appears
to be fine sand.  Interesting, the interior wood is mostly lined with
a blue paper.  Insect protection(?).

The rewards are worth the rebuild.  The tone can be sweet, assertive,
and very loud when all the complex swell shades are open.  The Harmonium
Home Page has many midi songs (classical and popular) written for
harmonium:

    http://shift.merriweb.com.au/harmonium/harmon.htm

Bill Chapman