If you wanna go the Deep Sky AP way, the 12" SC you're thinking about is not the best way to go. Way too much focal length for beginning, not to mention massive mount issues.
You could be looking at getting a big eq mount on which you could mount a big tube when doing observations and a pair of small EDs when imaging.
Another way to go is: 1. Getting a big dob (GSO, bintel or similar) and getting a good mount + a 80mm ed for imaging.
But starting deep space imaging with 3000mm of focal length is suicidal
