I'm the one who replied in your previous post about this, so might be a bit redundant but...
I would not go bigger than 6" on your mount. I like f4 scopes, they are fast and despite more of a challenge to collimate than an f5, I think it is worth it for the light gathering. If you really wanted to get something aroun 800mm focal length, you would be pretty close with an 6" f5.
What would you gain by sticking with a 6"? Better guiding, longer exposures, slightly wider field of view, less impacted by wind and less frustration.
What would you lose by not having 8"? IMO, not much. You lose a little bit of Focal length, but you also lose the frustration of having too big of a scope on too little of a mount. With a DSLR you are probably going to want to take exposures in the 4 to 8 minute range with an f4, and I don't think I would be happy with the results and quality of stars/image if I tried to do long exposure with an 8" on that mount.
I'm using an Atlas, which is the next size up from what you are using essentially, and with my 8" reduced (so I image at f2.8) and implementing a ultra low noise cooled CMOS camera, I don't like to push exposures past 3 minutes. I think I could get to 5 minutes, but it would require some more tuning of my setup, and with the big surface area being sensitive to wind... I need calm air to make it all work.
EDIT- I will add that with a firm budget of $500, a basic 6" scope will hold collimation and be easier to work with than a basic 8". I think if you go for 8", carbon fiber is really a HUGE improvement over steel... but at a price. Also, keep in mind you need a coma corrector, and this could cost another $200 to $300. You can have a 6" steel imaging scope and Baader MPC III coma corrector for about $500 here in the states. Not sure what you could get in Australia.
Edited by ChrisWhite, 04 August 2017 - 04:38 PM.