Tallone?
From '74 to...er...maybe 2000...not sure, BMW made air-cooled, opposed cylinder twins with disc brakes on the front. Manufacturers of the discs on BMWs have changed, but I wouldn't get one with drum brakes on front.
Displacement? I have a 1000 and a 900 cc. From the 750 to 1000 air heads (no radiator or oil cooling), I've found the most pleasant and reliable power range. Smaller than that and you have to ride at higher rev's, which I find annoying. Within that range, you can lope along at 70 mph with an ease and comfort level that is amazing...it is like the perfect combo of rpm, synchronicity, and power.
My bikes have 140,000 and 125,000 miles on them, respectively. They are not collector bikes, but daily users. They have carburetors, points, voltage regulators...you know, just like the pre-solid state world used to have.
Any air head with 50,000 miles on it is mostly, just broken-in.
My opinion, anyway. YMMV.
Have fun. If you like them, you'll love them.
(I DO admit to liking older twins with big jugs.
)
Kis