750cc and 800cc Airheads are among the smoothest and quietest "modern" BMWs out there.
My first BMW was a 1977 R75/7. I put 80,000 miles on it, nearly all trouble free.
If you get it, go through it, get to know it, tend to anything that is half assed or not quite right, then enjoy it for a decade or two.
Pastor, Church of the Horizontally Opposed Air Cooled Twin
'66 BMW R60/2, '83 BMW R100RS,
'81 BMW R100RS "Das Beast", '77 Honda XL125