The repair probably will cost more than the body... but I don't want to start fresh with a new body that could have problems.
Is the consensus shutter issues?
No! I wonder if the people who are confidently telling you it's a shutter issue ever used one of the original all-metal M645s like your M645J, or ever saw this problem first hand.
I have actually had this problem in one of my M645 1000s bodies. It's not a shutter issue. It is a light leak issue. I replaced the foam seals around the door of the film chamber, and that solved it. This is an easy DIY fix. A few bucks for a sheet of adhesive-backed black camera foam, cut strips to size, scrape out the old degraded black foam gunk, insert the new strips, and bob's your uncle.
Let's think for a moment about how this happens. If it was a shutter bounce, there would be blurring of the focused image in the affected area - but there's not. Instead, there is a very diffuse extended band of light - often bluish in colour (hint: light leak from blue sky around the camera). The light leak is happening not when the frame is in the gate, but just prior to that, when the frame is sitting around the top of the roll, and facing the back door of the film chamber (hint: the band of light has a sharp cutoff on one side, where it emerges from contact with the roll, and a fuzzier edge on the other side, where it curves away towards the front of the camera). It is all perfectly explicable by thinking about the geometry.
Ray