Get a qhy8L or a qhy9 mono and get on the ccd train. I would go a mono qhy9 now after using the osc just because you loose detail with a Bayer matrix also cal is less troublesome. The CDS-1100D is one hell of a nice dslr though but the shutter life worry's me and the sensor being cmos has it draw backs, but all in all its a dam fine cam and offers some mp's.
People over look bias readout noise with dslr's as well and that is the main killer on faint targets, put thermal noise on top which isnt linear unlike a ccd and it gets worse. There's a reason why you don't see dedicated high-end astro cams using cmos sensors. I would not mod a 6d because i have destroyed 3 1100d's already with mods, i got lucky with the first and the rest ended up badly. It's up to you though could end up in tears, unlike cheaper cams which only cost a few hundred bucks.
Why the hell you would buy a $1500 dslr for prime focus astro work is beyond me, its like buying a hammer to cut wood besides CDS dslr's ect which arn't going to be taking family pics with. Anyhow the qhy8l is a great cam to break into the ccd world with, they have improved read noise by pushing the board back from cooler and tec over the qhy8 as well. Anyhow on your budget the qhy8l makes perfect sense, I have a starshoot pro v2 the same chip and it gives nice results apart from readout noise which i am resolving atm. Take a bais from your dslr and subtract a stack of bias averaged from that and see what you end up with
heres a pic of my readout noise with the cooler off with bais - bais avg theres only about 5 pixels in the whole read-out reading anything.