I would do a min of 3" from CAT to Muffler, dual 2.5 out of the muffler would be fine. I would have them build something from CAT back if you want a custom. I would suggest a Delta Flow series, you will avoid some drone and just normal noise, but louder when it is opened up. Ignoring muffler cost I would say $300 for pipes, install, and welding a catback is reasonable.

As a nit, I would also prefer chrome tips, they just look nicer.