Will need the UPR k member if its the forward facing header kit. You'll a complete fuel system overhaul, the T-5 will need to go bye bye for an A5 or a T-56 (or an auto which is the best option for a turbo build). Suspension and chassis bracing is a given... Pretty much most of the rest of the car is going to need to be upgraded to make it work.
Remember, the On3 kit itself is just the hotside/coldside/turbo, its not like the Vortech kits that come with EVERYTHING. That said, the kit itself is very good and the quality issues were resolved a long time ago.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.