Just a minor point, but the R850, 1100, 1150, 1200 range of BMW's are not air cooled, they are Oil cooled. The oil in the engine has a dual role, it lubricates, but it also takes heat from the engine and then is pumped to an oil cooler to get rid of it (just like the coolant in a liquid cooled engine).
I can't add much to what has been said. Air cooled engines are fine, much of the drive for water cooling is noise regulations, the water jacket quietens the engine down considerably, the down side is that you have a water jacket running round your engine which can lead to head gasket leaks etc. (especially if disturbed).
I'd agree with R100R that the jump to a 1200 may be a bit optimistic - but if you like the R1200R you could consider the R850R, which is Oil cooled, the R80 which he suggested is the old air cooled 2 valve engine. There are a lot of bikes out there that may suit your needs, including the F650, but it will really depend what you want to spend, how confident you are, whether other things like shaft drive against chain etc matter to you.
Take a look around and sit on a few to see if comfortable with the weight, don't worry about the cooling, air cooled engines are fine.