What I did with my D600 was, I set it to small image size and jpg, and the shortest shutter time possible, and then I clicked away some 3000 exposures. It only takes a few minutes. After that, the oil problem was greatly reduced.
I still got the D800 after that, because I didn't want to have to clean the sensor during my vacation. In the end I was glad about it - the D800 has several nice things the D600 does not have. For example better AF. Also the preview DOF in the viewfinder works better (due to the viewfinder actually being coarser, I've been told, go figure), and it has a lot of nice features and button that make you faster in changing settings on location etc.
I don't need the 36MP, but now that I have them, I'm not unhappy about them. The detail this camera produces with good lenses (and your 50mm is such a lens) is indeed incredible.
I have the Nikon 50mm f1.8 G, the Nikon 85mm f1.8 G, and they are both phantastic lenses. For 35mm I got the Sigma 35mm f1.4 DG HSM which takes beautiful pictures too (although you should buy it in a shop because not all copies seem to work well with all cameras, series variation maybe).
By using primes, you'd get top IQ for less money, and could get the D800 without spending too much more money than you'd spend for top-notch zooms for the D600.