Well I hope that the ones for sale are in a little better shape than mine was.
Let's see. The legs can be in bad shape (mine had none worth saving) The warming ovens and back splash are prone to rotting away (mine was crushed by the roof & rusted) This is why my photo shows the stove sitting on a brick base with no warming ovens. The large cast plates inside the firebox will warp and deteriorate. Mine has a spot for
water heating but was never installed on this stove. I would expect issues on an old stove that has that option. General rusting. most all would need painting / oiling / new nickel on the shiny parts.
I'm sure that you can either restore your pieces or purchase / exchange yours for redone ones.
Any old monarch in really nice shape is now an "Antique" so worth more than a thousand dollars... I'm sure you can still find one for $500 or so with, poor nickel , rusted body , 3 good legs and a fake one. But it will still crank out the heat ! Good luck on your search, let us know what you go with.
Couple of photo's inside my firebox showing the warped plates.