The seller's price is just too high for what you'd get. Locating the missing 850 rear suspension is the easy part. Then you'd have to cut out all of the "custom" rear suspension crap that's in there now & repair a few broken items before you can reinstall those 850 bits. Then there's bodywork, which on this car would be a lot due to nearly every panel having body damage or rust issues (or both). Then refurbing the sun-baked interior, which is no easy task if what you have to start with is already crap (good Coupe interior bits are scarce). After doing all that, you'd basically have a straight 850 Coupe shell that still needs mechanical refurbishing & is still minus a drivetrain.
With the purchase price + body/rust/interior repair, you're deep deep underwater financially before even sourcing/obtaining/rebuilding an OT1000 engine for it. Yes, you could just obtain/install a regular 850 (or PBS conversion) engine/trans, but if you're going to do that, it'd be far cheaper to just find a regular 850 Coupe (maybe complete & in better physical shape) to start with. It just doesn't make sense financially, nor authenticity-wise. If someone's after an OT Coupe & going to pay the $$$ for it, they want the whole package, not just an emblem-converted 850 Coupe.