If you are looking to keep the prodrive backbox and standard downpipe, without modifying anything it's a 2.5" centre section that you would be looking for. Most aftermarket downpipes are 3" but you can get a reducer if you have a 2.5" pipe.
I got a great deal on the brembos. £350 including brand new PFC pads which are about £100. Had to buy disc (went for Godspeed g-hook £250). Most calipers I saw were about £500ish. I think the brakes are great. Make a huge difference. I'm sure I saw on here that some had a set of prodrive Alcon calipers for sale, they would be amazing if still available.