Guys, please don't run ahead, or the thread will degenerate into the usual obscure tangle of opinions and we achieve nothing.
Let's see if it is possible to cut through the chaos and come up with a step by step procedure.
Remember that at this stage we are only have focal length to work with and we have to choose the astrograph type, not the make/brand. We can deal with those issues later, in a systematic way.
I propose that we cut out anything below 400mm FL, as that range would just generate a messy discussion of photographic lenses.
If we use exponential series, I think we can get away with three domains: 400 to 800; 800 to 1600; 1600 to 3200.
So we need to choose the best astrograph type for each range.
These are theoretical astrographs at this stage and are meant to be capable of producing near perfect fields that cover a full frame sensor.
Sorry Peter, your setup is out. People that use extreme gear like yours, don't need this kind of help.
But your input would be appreciated.
Knowledge of optical systems is very helpful at this stage.