I vaguely remember talking to you many years ago now about FX?
I'm mainly shooting the D700 now. The only reason I shoot the D300 is for wildlife on very long lenses, something you are not doing. We have a lot of commonality in lenses...
You may very much like the 85 1.4D on FX. I think the 85 really shines on FX because you can get closer, and at that focal length I think closer is better for DOF reasons. I suspect you will also like the 70-200 for much the same reasons.
As I understand it, the various high end 85's have always outsold the 135 (historically in the film world where everyone shot "FX"). I believe there is a reason for that and why I stress that lens, which you already own. And I also recall admiring many of your images shot with that lens, especially the street portraiture you've done with it. Of course, you could be a "135 FX" kind of guy, in which case DX works well for you.
I have the 20/2.8 Ai and the 12-24. The reason I kept the 12-24 is that it would allow me to shoot two bodies, and use DX for ultra-wide. I have the 24-70/2.8 AFS, which makes me not as excited about primes because I just love that zoom.
I've never solved my ultra-wide FX problem. I looked at the 12-24 on FX, because like you I heard it was "usable". Personally I do NOT consider the 12-24 very useful on FX. The usefulness depends on how you feel about smearing in the corners...
The 12-24 adds a severe compositional problem, where I have to look at an 18mm image, for example, and try to visualize exactly where in the frame the smearing will be objectionable, and frame accordingly. Not that it can't be done, it just isn't what I call fun. And I found it easy to blow an image because something I wanted in the frame when I shot the image was not acceptable on critical review.
Rather than go through that, I just use the 20/2.8 Ai or shoot the 12-24 in DX mode and live with 5mpx (D700). You would be "living with" 10mpx on the D600 so I think that approach is far more viable for you.
There is not much of a difference between 10 and 12 mpx DX. You are now accustomed to 12 mpx DX, so from that point of view the D600 *could* be used as a sort of hybrid camera, and buy time for you to complete the lens upgrades over a couple of years or however long it takes.
How you would feel about all this depends on what lenses you shoot now and when, and for what...
If you rely on your 17-55 now for a lot of your shots, and you also frequently shoot it wide open you may not be happy until you have a 24-70 (just as an example).
On the other hand, if you enjoy working with primes, you should almost certainly find the FX viewfinder far easier and more enjoyable with your MF lenses, and even with the AF fast primes I think you will find that you have more confidence in the camera's focus. I was never comfortable with my MF lenses until I went FX. I'm even reasonably good with my 50/1.2 Ai-S wide open.
All the "why do you want to do this" concerns raised here are valid, from my point of view. I'm just trying to add a few angles maybe not well covered, and mainly to point out that 10mpx DX isn't all that terrible as a transitional aid. You already have a lot of good FX glass; you mainly have an ultra-wide problem.
If you want to explore the 12-24 DX vs 20/Ai-S issue further I could concoct a quick and dirty real world shoot-out to illustrate relative usable FOVs. I know this is a confusing issue because there are so many varying opinions and I think that is all based on varying personal expectations of the corners.