I owned a couple of the metal Toyo half plate cameras with the interchangeable backs. I had the 4x5, half plate and whole plate backs but never saw an original 5x7 back anywhere. People spoke of them and I kept my eyes open for several years but never saw one, never mind one for sale.
Lo and behold a camera with a 4x5 and the rare 5x7 back popped up on eBay recently. I watched the auction out of curiosity and it got a few bids and ended at $380 USD. Someone got a heck of a deal IMHO.
Yes, although they will have some work to do. I was fortunate recently to acquire the same model with both 4 x5 and 5 x7 backs. I paid more but the camera was in almost mint condition. The last owner had not used it for 15 years and also had a Linhof board adaptor as well. Needless to say, it has been put to work since its arrival. The only maintenance it required was some lubrication of the track and back slides as they were a bit stiff.