This may sound like a ridiculous idea, but I'd buy a second hand APS-C canon, and astromod it yourself. Just rip out the filter totally. This will cost you under $200 in total. I think my 550d cost me $130. Then whatever you'd like to do with fitlers... clip in or 2" if your focuser/t-ring/flattener has threads for one? Or a 2" filter drawer added to the mix?
I'm not sure what IR/UV cut you need or want in that scenario. But with money saved you might be able to get one of the new duo-/tri-/quad-band filters which would be pretty awesome.
It will mean that your image FOV will be smaller, and the flattener should have less problem covering the field.
Then get guiding going
Rationale here is that I think you can take a modded dslr quite a long way, but its hard to get far without longer exposures away from the pole. And you get to tinker.