I'd say it was an '84. The '84 has a rear brake cable stop brazed to the seat stays, and the '83 has a cable stop that hangs from the seat post binder bolt.
The rust inside the frame doesn't look too bad to me. I think just coating the inside with frame saver or something similar would be adequate. Touch up paint might be kind of tricky to get a match but a lot of people use Testors model enamel, mixing different colors to get a match. Or you can bring the fork to an automotive paint supply store and they can usually mix up a match and sell you a small amount for about $20 or so. You also need some primer.
I have two '85 610's, one is the same color as your's and the other is what Miyata called burnt sienna. They are well built bikes with nice components.