mabee consider an 8 or 10" dob if you plan on using it for deep sky imaging on an EQ6, anything bigger and you need to throw some serious money around to drive it reliably enough.
i started imaging with a 6" newt on a HEQ5 and i couldnt imagine being a beginner and starting off with an EQ mounted 12", you would probably end up throwing the whole lot over your neighbours fence half way through your first go
if you have your heart set on the EQ mounted 12" though you will need to get a Losmandy G11 instead of the EQ6