Retrofitting a TV into this unit shouldn't be too difficult. A flat panel should fit in the opening very well, with room to spare. If you want to put a standard CRT set in it, though, be careful that you allow for the two or three inches the CRT cap on the back adds to the depth of the set (this applies to today's BPC sets as well as vintage ones with masonite backs and plastic tube caps). I don't know how large the original set was, but my best guess is 19 inches. I occasionally see these upright Maggies on eBay; most of them are 19s. If you don't want a TV in it, the extra space could be used for record storage.
I can't imagine what could be wrong with the phonograph.
The changer looks like it's in fairly good shape, but then again it's hard to tell with such a small (and non-zoomable) picture. Could be any number of things, as simple as just dried-out grease in the changer to more serious problems with the amplifier.