The serial numbers starting with 5xxxxxx are Canadian models I think.
My D600 is 5000XXX. Before I bought it I rented a D600 with the serial number 5005XXX.
Both cameras showed evidence of dust. With the rented camera, I used the Giottos rocket blower which removed a lot of it. Only the very small light coloured dust remained and it didn't show up in any photos.
The D600 I bought had less visible dust and most of it was removed with the blower. I also did a cleaning with a wet swab from Visible Dust.
I am monitoring the sensor and it appears that some dust continues to accumulate but it's not a great amount and not noticeable on any of my shots.
I shoot at f/8 and lower almost all the time so the issue is not bothering me right now and I will clean it again if the dust starts to show up in photos.
The dust is not a deal breaker for me. This is a superlative camera. So I would say it is safe to buy if you can accept a little extra maintenance work.
By the way, I did the "dust test" on my D7000 which I would have said was pristine and it showed quite a bit of dust too. I think many of us have been using dusty cameras without knowing it.