another 5 cents worth Andy - most of which has already been said.
You don't need a big scope. The linked composite image shows M83 taken with scopes from 8.2mVLT aperture down to Codemonkey Lee's 0.12m Chinese refractor. The big scopes do better, but the VLT clearly does not produce 5000x more information than Lee' scope - the differences in detail can reasonably be explained by the seeing - 0.7arcsec on average for the VLT and maybe around 2 for Mt Lemmon and Lee's system. And of course, nothing comes remotely close to Hubble with no atmosphere.
http://astrob.in/314502/0/ .
ie, the atmosphere is the primary resolution bottleneck for scopes above about 0.1m aperture (unobstructed) and going bigger may increase sensitivity, but does not get sharper results in typical Australian conditions. A central obstruction changes things a bit, so you could aim for something around 200mm as a minimum aperture.
The focal length should be long enough for the pixels to extract all of the detail left in the optical image, after it is blurred by the atmosphere - with your CCD, about 1.5m will do the job in 1.5-2 arcsec seeing. You could go longer, but would not get significantly better detail - and you would need to spend a longer time to get an image (eg going from 1.5m to 3m would increase the imaging time by a factor of 4x).
Given that you might want something with >100 mm clear aperture (~200mm+ if obstructed) and 1.5m fl, that will work on an EQ6 - you could consider:
- an 8 inch f8 RC with field flattener (will take a long time, but give best resolution)
- a vastly expensive APO refractor
You could consider a shorter scope and use drizzle to recover some of the detail lost in sampling. Then suitable scopes would include:
- an 8 inch f5 Newtonian with coma corrector
- a 10 inch f4 CF Newtonian with CC (on the weight limit for the EQ6)
- a 7 inch Mak Newt (also a bit heavy)
Whatever you get will need a good digital focuser - my CF f4 Newtonian needs refocusing (by SGP) about every 10 minutes in typical conditions. The Newtonians will need at least a 3 element coma corrector and the Skywatcher (a bit better mechanically at this size) or GSO tubes will require stiffening around the focuser to hold your heavy camera - some sections cut from a cheapo tube ring can be glued and screwed to the OTA to do this.
I reckon that a 200mm Skywatcher steel tube f5 Newtonian (drizzled) with the Skywatcher CC and Moonlite stepper focuser should do the job and be close to your budget. The Skywatcher optics should be quite good enough out of the box. Sounds like a fun project.
Cheers Ray