Are you aware of the shutter grease spot issues with the D600? The D610 has unofficially been released to fix those issues.
If you can afford it, the D610 would be a safer bet as a result.
For lenses, you’d need to get a decent 50mm to replicate the effect of the 35mm you have now but as far as I am aware there isn’t a superzoom available for FX, that’s more of a consumer level thing. You’d need to replace the 18-200mm with a couple of different FX specific lens at least..
I’m looking at a cheap D7000 as a step up from my D90. I am sure that the D7100 would be a signicant upgrade on your D80 and you wouldn’t have any lens issues…