Questions...

[burkbuilds]

I rode my GZ about 12,000 miles the first year, mostly to school and back every day, but I wasn't going half as far as you were and the biggest vertical obstacle was a hill along the road that no where near qualifies as a mountain or anything and I almost always dropped 5 mph on that little hill. Now having said that, I'm 210 lbs and that's a lot more than you weigh so it might not be as big of an issue for you. Also, I modified my seat and had it recovered to make it more comfortable. Most GZ riders will tell you that they aren't comfortable on long rides with the stock seat, although I think Water Warriors wife rode across Canada with a stock seat and was fine with it. Even with the seat customized on my GZ, riding 100 miles was not comfortable. When I found out I was going to be travelling 100 miles on the highway each way each week for an extended period I sold the Gz and bought a Vulcan 500. The stock seat is very comfortable and about the same height as the GZ, right at 28 inches. The Vulcan will get about 10 mpg less than the GZ, but you won't have any problems maintaining any speed you can legally drive on any grade without downshifting. The Vulcan at 498 cc's falls just below the 500cc price hike that most insurance companies start raising their rates at.
AS far as holding up, the GZ will definitely hold up if you take care of it. As Easy Rider pointed out, all those guys who blew the engine early were running it wide open throttle for extended periods. I'd be willing to bet almost any cruiser that was ridden WOT for extended periods would probably blow in a similar number of miles.
As to comfort level on my Vulcan. I rode 2 1/2 hours without a stop Sunday evening, mostly on I-75 at speeds of around 75 and I was fine when I finally arrived and got off, no stiffness or "monkey butt" or anything. So I'll have to disagree with Sarris on the point of saying you need something 650cc minimum but I do agree that the GZ will probably be more uncomfortable than you will want. I don't have any problems with the bikes he recommended I'd just like to add the Vulcan 500 to that list as a bike that would be comfortable and meet the requirements you have listed. Good luck with whatever you choose.