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Size is absolutely important. Mostly the size of your wallet.
An f9 scope is going to more difficult to work with than say and f5 or f6.
Not that excellent results can't be obtained - it just takes longer and your guiding has to be all the more accurate.
If you are NOT constrained by a budget, then either a Televue NP101 or Takahashi FSQ 85 or FSQ106 (all around $4k) will give you a lifetime of imaging happiness.
So will a suitably reduced 8-10" SCT or Newt.
In fact, the Newt is probably the best option. Imaging Newts (at around f4) are now available from a couple of suppliers. They are inexpensive compared to refractors or SCTs and quite easy to handle.
An ordinary Newt at around f5 will do just as well and double as a planetary instrument, though you would want to add a coma reducer.
This site's head honcho "Iceman" is probably one of the top Newtonian imagers in the world. Seek his opinion!
If you want to stick to a refractor (they are quite sexy) and are on a budget, virtually any quality 80-110mm APO will produce excellent results when combined with a field flattener.
My sweet-spot would be a Sharpstar (Astro-Tech) 106 / f6.5.
At around $1800 with a flattener and or reducer, I can't think of a sub-$2000 scope that comes close to this instruments quality. It really is astounding.
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