Jim,
I was 17 when I got my new '74, only because the original buyer backed out of the purchase and I was first on the waiting list. I immediately threw on a giant carb and ported the cylinder, when I should have just left it alone.
I had a lot of fun with that bike, and still have it (it's a 175 now).If the '75 is on the original bore and the gearbox oil is clean, I consider these both fairly good indicators of low hours and (maybe) decent maintenance. I don't know how easy it will be for you to verify these, but I'd at least ask about them. My left-side main bearing cage fell apart, so you might want to check for crank play too.
Ray