Here's my take on BMWs.

BMW K75 - Sit on it and try it first! It has a TALL saddle height, non-adjustible. Smooth and bullet-proof engine, though. K75C is standard seating, K75S is a bit more lay-forward, sportier.
BMW K100 - Same frame as K75, a bit more power, a bit more vibration.
BMW R - 1970's 'Airhead' (air-cooled) vintage, durable, not quite 'modern'
BMW R - 1980's 'Airhead' (air-cooled) better brakes, durable, more 'modern'
BMW R - 2000's "Oilhead' (oil/air-cooled) a leap ahead in frame and motor technology with EFI
BMW F650 - liquid-cooled, carburetors 1997-1998 (or so) EFI afterwards. Very light, agile, competent on highway, but not first choice for freeway droning... 50-60 mpg, though... Dakar is biased toward off-road, others are biased towards street. All are competent on the street.

Past motorcycles I have owned:
1982 BMW R100RT - airhead, touring (sold with 30k miles)
1992 BMW K75RT - brick, touring (sold with 45k miles)
2002 BMW R1150RT - oilhead, touring (sold with 15k miles, bought K1200LT) [Fast, comfortable, fast!!! Good two-up, d****d good solo!!!]

Present stable:
1994 Yamaha Virago 750 - fun V-Twin, not enough spare $$$ to insure right now
1998 1998 BMW F650 - 2-carburetor 650cc single, a giggle and a half
2002 BMW K1200LT - brick, touring, 'Light Truck' (current two-up, me & Mrs.)

Try out as many as you can, it's hard to buy a bad bike...

Best Regards,
Jim