Ok ok, there a lot here, do you have a reasonable budget? Because I could send you towards some £3k coil overs or £1500 exhaust.
Also, with the breather mods, are you after power? Sound? Mix of both?
Intake - Takeda (sound and looks) OEM with velocity stack and high flow filter (power)
Plenum spacer - PM Wasso, he'll sort you out with shexay ones
Headers - PITA to do, but DC seem a good mixture of quality and price, or Stillen aren't too badly priced either.
Decats - personally I think you should go for Berk High Flow Cats, they're still MOT friendly
Exhaust - Invidia Gemini. It's a true dual cat back, nice noise.
ECU - Stock with UpRev tune. Cheaper than a stand alone ECU and includes a few different maps.
Coilovers - HKS Hypermax, BC, Meister R, HSD.
Camber Arms - SPC/Eibach. You'll only need rear ones if you lower it more than 30mm from stock height, fronts can maybe sustain up until about 40mm drop before you need to buy adjustables.