Ironically supercharging is probably the most cost effective way to a considerable increase.
There are Mustang based tuning companies out there that will provide all the others you ask for but costs could quickly mount up.
Brakes and chassis don't need any work to handle up to 400BHP although Andy Kitson has done a revised suspension package.
http://www.vehiclehandlingsolutions.com/page1.htm
Most owners who can't afford the SC bung in a K&N filter mainly because it won't need changing again and getting at them is a pain plus an accufab throttle body and maybe louder back boxes, induction kits don't seem to achieve much other than make a noise.
The headers are a tight fit and may be restrictive but if they are good enough for the SC setup then no point changing them.
The standard Ford ECU has a Rousch tune in it so there may be some alternatives out there but so far every petrol chip tuning product I have ever seen has been too much money for not enough extra BHP imho!
Strangely enough if Phils claims are correct adding LPG may offer a BHP gain!