The issue appears to be settling down. That's not a guarantee, but at least the odds are much better of getting a camera with a clean sensor. I bought a refurbished D600 in February with no issues.
Just make sure you buy from a reputable dealer that has a good return policy. Then make sure you test your camera when you get it. Some of the dust issues can be fixed by using a rocket blower, but an oil issue needs more attention.
There's a good post at the top of this forum about sensor issues and a ton of information on Google about cleaning camera sensors.
Good luck. It's an excellent camera.
"Red is gray and yellow white, but we decide which is right ....and which is an illusion"