Lot's of great advices.
If you do not mind Alex, I am happy to share my take on your choices.
If you are going with 1600MM, then 1000mm FL is around optimal for imaging DSOs, and with 16200, around 1500mm FL should give optimal sampling for multi-night imaging of DSOs.
With those focal lengths in mind, for DSO imaging, I would look for the fastest scope I could afford. So either a quality 10"RC with a quality reducer, or a quality Newtonian.
Last but not least I would consider either top mount that would not hinder the resolution (what's the point of getting a huge scope for high resolution long focal length imaging if data gets blurred by the mount?), or a decent mount but with a camera with Active Optics.
Good luck and please keep us updated