Good news - BMW airheads are hugely interchangeable, and for many years. There are of course catches and gotchas, but essentially all /7 and later all the way to the last airhead in about 96 or so are the SAME engine transmission package, and all are interchangeable. R80 and R100 top ends are completely interchangeable.
Cylinders, rocker covers, air cleaners, carbs etc have changed, but as a rough guide - bikes from the same era, e.g. all /7 and all 81-95 are extremely similar and interchangeable. E.g. an 81 R65 gearbox will fit right into a a 95 R100R, and with a couple of parts updates internally, IS the same.
The frames are even essentially the same.
The bad news - unless you get a super deal on the bits, converting bikes ALWAYS becomes far more expensive than you think, and rarely really works as well as the factory build.
My advice - get the best bike you can within your budget, leaving yourself at least £500 for repairs and maintenance items, and ride it. An R80/7 in excellent condition wil go around the world just fine - it doesn't need to be converted to a GS.
An R80/7 engine will bolt into a GS. NO significant mods required.
Most conversions are done by people that already have a bike, and decide it isn't quite what they want, so they start modifying it. And end up with a bike that's cost them twice as much as if they'd sold it in the first place and bought the right bike - and it's rarely as good anyway.
You're starting from scratch - get the right bike for what you want to do - if you're convinced you need a GS, then get the best one you can find, maintain it properly and service it and ride it. Don't start modifying until you really know what you're doing and WHY you're doing it.
And yes, I've built a few $pecial$...
Seek, and ye shall find.
One world, Two wheels.