If you are really after the "execellence on mechanical technology", I can only repeat my Leicaflex SL reccomendation. It really is a step above the F2 and the F.
The SL2 is even more of a mechanical marvel, but would be over budget.
As pointed out, lenses will usually be more expensive than Nikon ones, though not as much as legend has it.
Re Spotmatic: lovely camera, but viewfinder is a bit dark. Also, if you change lenses often, screw mount is a PITA.
Also, depending on which lenses you're using on your Contax and Rolleiflex, you might find Nikon lenses to be a step down, though if you choose very wisely, that can be minimized. Unfortunately Rolleiflex's mechanical SLRs (SL-35 and SL-35M) are neither jewels nor tanks. While not a mechanical jewel by any means,your SL-35E is actually a very sweet camera - as long as it keeps working. I use mine a lot!