It's a bit like real estate.
Mounting-Mounting-Mounting would be the three rules.
CCD/CMOS are linear devices...well, sort of with the latter, so image depth is purely a function of how dark you sky is, and how long you want to expose...but you can cheat a tad here and put a $13k 97% QE camera on a 4" and, as the Stones lyric goes "make a grown man cry"
Resolution? ....sorry, aperture rules....and can make the first rule expensive.
It is all about balance... just accept you can not do it all with one focal length, look at the images that you would like to emulate, then write the cheque
Cheers
Peter