Take into account that old Model Ms tend to have their plastic rivets fail: those keep the sandwich plate-membrane-plate together, and after enough of them break some keys stop registering. There is a laborious fix, called “bolt-mod” which consists on drilling the rivets off and putting bolts with nuts instead.

So, you either get one whose keys have all been tested to work, or you should be ready to do some manual work.

As far as I know, internally all models (M122 terminal, standard Enhanced Keyboard, and Space Savers/TKL) are internally the same w.r.t. reliability, noise, and feel.