If you search for some of my responses to Astrophotography and what to use i have written a lot.
-Flex isn't a issue with my dob and the Orion/Skywatcher dob are just different colours...
-You will need a EQ6 platform for the weight as a EQ5 will die in the A55.
- These dobs have not got a long focal length, they are actually the fastest dobs in Syntas range of dobsonians at F4.7 and they are affected by COMA...
- Gstar will be fine for doing Planetary work, but DSO's not so well (not to say it wont capture them)
- with 10" of love get a 5x Powermate coupled with a DMK21 and that will be a powerful one shot planetary camera
- the use of a DSLR - Canon 350 modified (can be purchased for 4-600) will produce spectacular shots.
you will also need to guide as DSO's need decent exposure times depending on your setup.
Astro isn't a easy thing to do, your best bet is to take yourself and your scope down to a astro group and find the people who have gear like yours. you will be surprized by what you see.!
Have a look at the photos i have taken that are on the website in my signature thats what a canon 40d (just more Megapixels) and a 10" can do.
Brendan