I don't have direct experience of that model. But it sure looks like an ETR with a modified back, nothing more. So I would read up on the ETR, issues and problem and relaibility. And assume that the ETR-C will be pretty much the same?
My use of an ETR never involved switching backs, so an ETR-C would have been fine for me. The thing to watch for with any camera of htis age is a combination of old seals and gummed up grease, with possible hard use. They aere popular cameras with wedding photogaphers so many have been ridden hard, so to speak. Then again, an old camera that hasn't been used can be gummed up, etc. Check the battery compartment, and be sure to test with a fresh battery.