hi PRB, welcome to the nut house.
I can't (and I think nobody can) tell you what's best for you, but from your description, it looks like you wouldn't do a mistake buying the C14. It's a great bike for those things that you're describing, and based on my ex- (5' 4'') and my current (5' 8'') girlfriend, they prefer the C14's rear seat over that on the K1600, the R1200RT and the FJR. The reasons they mentioned were seat comfort and seating position on the two beemers, and the wind blast on the FJR. Apparently the C14 offers the right combination of seat cushioning, leg position and visibility-vs-wind blast. Sorry, no experience with the K1200GT, but I drove the 1200VFR and it's really good, I just cannot stand the stupid automated gearbox and I don't fancy touring without an electrically adjustable windshield, plus it's simply expensive.
The one point in which you should focus when test driving a C14 is the bending angle of your knees, since you seem to be a tall guy.
Performance wise, you'll be blown away. I drive the BMW S1000RR for at least a couple of thousand miles every year and I tried the ZX14R, so I know what a fast bike is, and the C14, while two leagues below them, is still pretty quick.
What to look for when shopping for a used C14? I don't know. Mine is 6 years, 60 000 miles old, and there's nothing wrong with it. Tires, oil, fuel, and soon the second valve check. That's it. Recalls have been made, and I would sleep sound if I sold it to my best friend.