[njsax]

You're overthinking this. Is this a science project? Buy the bike that begs you to ride.

Any R1150xx is going to be 22 years old or older. They're gonna have problems, like the Clutch Slave Cylinder puking all over your Clutch and contaminating it ($$$, and not simple). Early versions of those 1150 bikes have ABS (ABS-II). Servos came on to the scene around 2002 (I-ABS). Don't want servo? Buy model year 2001 or older. Consider R1100xx. The factory Shocks are only good for about 30k miles. After that, they are toast. You'll need to replace them ($$$).
Brake maintenance is key to problem-free operation, including ABS-II and I-ABS. If the seller cannot document regular and proper brake maintenance, walk away. Stay away from bikes that have had a K&N air filter for the past 30k miles. That engine won't last much longer. Stay away from bikes that have had a lot of aftermarket electrical add-ons. Often, the electrical connections are poor and prone to failure. These vintage BMWs had Euro-compliant, bio-degradable wire harness sheathing. After 10 years, the wire harnesses start falling apart. After 20 years, the wires are no longer protected and you'll have to take apart the bike to re-wrap all the wires.
Heated hand grips are nice to have. Make sure they work on the bike you're considering ($$$). If the fuel vapor charcoal cannister was removed (common modification), ensure the vacuum lines are intact and not leaking.

1150 series - 6 speed trans
1100 series - 5 speed trans
both are OK

If you're looking for a "standard" motorcycle, which is what the R1150R is, then you might want to consider one of the new retro Hondas or Kawasakis. Reliable, better handling, and more power than your old '78 Honda. An old BMW is really not for the casual rider. But if you're the kind who would rather wrench than ride, go for it.