If you want stage 2 then you only NEED a Performance downpipe which will cost approx £500 the rear cat back section will produce negligible horsepower gains and isnt required at any stage of tune. Depending on your software you will have to pay for the software to be updated to a stage 2 setup, as a rough guide this is £50 for REVO, Free for APR, £100 for Shark Performance.
An induction kit ISN'T needed. But it will help unleash another 10-15Hp. This cna be done at anytime as the ECU will adapt to this on its own, so no software tweaks required.
An intercooler ISN'T needed. But it will help unleash another 10-15Hp. This cna be done at anytime as the ECU will adapt to this on its own, so no software tweaks required.
Typical figures
Stage 2 map + Performance downpipe; 250-260Hp
Stage 2 map + Performance downpipe & Intake; 260-265hp
Alternatively
Stick a K04 turbo and S3 injectors (£500 ish) on the car keep the stock intake, intercooler and exhaust, then get a custom map and you'll be seeing 280-300Hp.