DISCLAIMER: I am NOT a mechanic. I don't even play one on TV.
I would still pass.
First, find the post asking people how many miles on their bikes they have. You will see insane, Gold Wing like numbers of Miles/Km's with little more than tires, cables, chain/sprockets, brake pads, and liquids. Based on the roadworthiness of these bikes, coolant in motor for any reason would make me nervous. Especially at 12K.
Even with the dealer offering a guarantee, 300 miles is material if you need something done. Heck, my dealer is 75 miles away and that's a pita.
If it were my 4500.00, there are Versys out there and available with stellar maintenance documentation and care........not that this means that you couldn't buy one and it wet the bed tomorrow.
At a far cheaper price with a 2 year motor warranty, maybe.
Finally, I tip my hat to the dealer for being as honest as the day is long. They could have quietly fixed it and the future buyer would be none the wiser.
Good luck with your decision.
2014 Versys 650 in glorious Kawasaki Green