

There are many seasoned riders here and they will offer their input for sure but because I read some reviews, I think I have a few things to say.
Yamaha 650 may not be the best choice for long trips on the highway. They found the engine too busy on the highway, the front suspension was not good either. I just can't understand why on earth, Yamaha 650 doesn't have shaft cover!!. Don't recall any reviews on Kawi 500 but perhaps one could put it in the same class as Yamaha 650 so its power for highway riding may be questionable.
Having said that a guy next door rides the 500 2-up, guess in town it's quite alright.
I ride a C50, am slightly shorter and weigh a little less than you. Considering your prior experience (albeit long ago) and height/weight/physical condition, I should think you could handle the M50. A big plus with C/M 50 series is that they're all fuel injected. If you want a lighter bike, check out the S50, it doesn't have the latest technologies, but I personally love those bikes, proven old technology, light and fast. And great looks too. There are members here who swear by their S50s or formerly the Intruder 800.
I love the Kawi 900, too. Notice that it's got all the bells and whistles; belt drive, rear disc break, positive neutral finder, fuel injection, great styling and colour schemes. In terms of weight, not too much heavier than the C/M 50.
A friendly advice: Get your MSF training first, in the mean time dealers will introduce good deals to make room for the upcoming year's bikes. You could then test ride your favorite bikes and make an informed decision.
If I were in the market to buy a bike, I'd test ride the following bikes: S50, M50, Kawi900, Triumph America, Shadow 750-1100, Yamaha 1100-1300, VTX1300, H-D Street Bob (started to dream, better shut up now

).
Best place to get info on M50 is
here.
Good luck!