I don't know where that '3200 hours and it's no good' info is coming from, I've been reading for years that an Onan generator is designed for 10,000 hours before a major overhaul is needed. I'm pretty sure that's in my owners guide too.
If your genset was running fine, and just stopped? Mine did the exact same thing. One of the end shaft bearing balls in the ball bearing broke and boom! engine stopped dead. Shop in Portland had it out and replaced for $640.
You don't tell us much about what happened before and after it got to that point like did you try to restart it? What happened if you did? That could help with a more accurate evaluation.
But if it were me, I'd remove my generator from that shop and tell them thanks but no thanks.
Here's the goto guides for analysing your Onan's symptoms, very handy resource with their complete testing guides:
Onan TroubleshootingIt's odd that you now have to dig three pages deep in Google search to find them now days when they use to be the 3rd one down back in 2005. All those other companies paying lots of money to be higher in the search results.