The various F2 models are the same body (or essentially, so, some small tweaks may have been introduced over the years), with the prism determining the model.
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!
An EX+ condition F2 will be over budget, one with a non metered DE-1 is about 5 times the SL2.
FM2 is probably the best choice