Not that I imagine many of us are likely to, but in case you were thinking about a D600, you might like to look at this:
http://www.petapixel.com/2012/10/22/the-nikon-d600-has-sensor-dust-issues/
Probably just a bad batch from the factory that the hire place has unfortunately ended up with, but worth keeping an eye on if you were to take the splash.

The Nikon D600 should be the perfect enthusiast camera with it’s fantastic specs, great low light performance, and an affordable full frame sensor. However, many reviewers and blogs have been raising concerns over a severe dust issue. Dust has reportedly been collecting on the upper left corner of the sensor at an unusually high rate.

Of course, only those with a problem will speak out; if they sell a million cameras, and 99% of them are fine, that’s still 10,000 people hitting the blogs to raise hell over their defective products.

Is the D600 dust issue as bad as the critics claim?

I thought I would put this claim to the test by purchasing a brand new D600, putting on a 50mm 1.8D lens, and shooting some test shots, before using the camera for anything else.

And, from that I’ve seen, dust on the D600 is indeed a serious issue. Right out of the box, after taking the very first picture, I could see several dust spots, but as my time-lapse series progressed, I could see more and more spots appearing on the sensor. Keep in mind that I am not changing lenses; all this dust is coming from inside the camera.

Where is this dust coming from?

The D600 is a great camera, but do yourself a favour and pick up an extended warranty to go along with it. Trust me, you will be using it.