I'm looking to purchase C3 fender flares and as i'm rebuilding my rear suspension i'm going to get 2" offset trailing arms.. How wide (max) can i go as far as wheels and tires with backspacing???

As good as the tires are now days, you can probably get as much traction from something that can fit in the fender well f/ take-offs or cornering. You can even go w/ lower profile tires and bigger diameter wheels.
I do realize that the big fenders and wheels give the car a whole different look.
Good luck w/ your project.