Hello all, longtime optical designer here.

Klaus - why an Apo El Nikkor? You get almost the same performance from an f/9 Apo Nikkor at 1/20th the price. The only significant difference is that the Apo EL's starts optimizing one stop faster, around f/8, while a 4-element symmetrical airspaced Apo starts to optimize around f/11 on down. I have a whole set of these; they're incredible for enlarging or repro work; better than conventional enlarging lenses. but they can also be used in a view camera for extremely well-corrected overkill applications anywhere from macro to infinity. Apo El's were once coveted for high-end scanning cameras used in art reproduction and forensic sleuthing; but now those come with lenses of their own.

I could have once purchased a 210 Apo El at a rare reasonable price. But as it is, my own Nikkor and Rodenstock Apo's are almost too much of a good thing, capable of revealing even the smallest flaw in an emulsion or piece of carrier glass.
For that reason, I also keep around a set of regular enlarging lenses as well. But for generating very precise enlarged internegative or interpositives, the Apo Nikkors are the way to go when focal lengths 180mm or longer are suitable.

Well, it's nice to have another optical engineer around. Two others chime in from time to time, though more often over on the LF Forum rather than this one. I worked side by side with an ex-Nasa optical engineer for a couple of years in a machine-shop supply role. Then one of my own long-term customers was the owner of the most advanced aspherics company in the world. They made the correction lenses for the Hubble, for example. He has passed away, but the remaining facility is still a prime producer of astronomical optics. They're about a ten minute drive from me, and will make a lens or mirror for anyone, provided you have a NASA or NSA or EU Space Agency credit card without a limit on it.
Don't carry that much cash, or the same authorities who went after Pablo Escobar will come after you.