Being new to bikes, the OP isn't going to know what to look for in a 70's UJM. Given the popularity of them, you've got people dragging them out of barns and garages, cleaning them up a bit, and getting decent money for them. Unless the OP has a friend that knows these bikes, he could easily wind up w/ something that may look good, but have hidden issues. That's not to say that that possibility doesn't exist w/ a 90's bike, I just think it's lower.
If the OP could find a nice 70's CB450 or XS500 in good shape, that's well maintained, I'd certainly recommend buying it. There are just less of them to pick from than a mid-90's CB500/GS500E/SR500 in good condition. And the price is going to be about the same. I'd probably go w/ the Honda or Suzuki over the Yamaha, just because they're twins rather than a single. I think either of those is a great first bike, and should
have less issues than their older brothers.