Look what followed me home. . .
This one found you, and it sounds as if you're the right guy to bring it back to life. Once the sting of the misrepresentation passes. . .you're in for some rewarding fun -- you clearly have the skills to make this one right, and the thrill of having a smooth running creature that brings back memories. . .priceless.
[I]Attitude[/I] toward a project bike is everything, and unless you REALLY got soaked on the price, just accept the fact that this is going to cost a few bucks to get right, and also know that it WILL be right one fine day. Forget about economics, and just go for it.
You've got a lot going for you with this one, mostly that you have a real connection with the model, and -- as you know, that 750/6 has a lot of fans. "Almost" anyone purchasing a used bike (especially from afar) has got some work ahead, and usually some parts purchases along with the garage-time. Sometimes you get lucky, and sometimes not so much, but from your description, this is far from a total disaster. Trust me, you are not the first guy to buy an old machine that needs more TLC than was represented -- DAMHIK.
The 750cc engine gives you some "room to work" if you have to go slightly oversize. IF that's necessary, MotoBins in the UK is a great place to shop for some of the big bits.
+1 on Max's as parts supplier. Rusty in the NH store is THE man when it comes to the parts for airheads -- we're very lucky that SO much stuff is available for these old marvels.
Plenty of help available on the Airhead forum, and you might look into membership in the Airheads Beemer Club.