Well thats helpful when NO-ONE has ever got 300hp from a Cupra TDi before
You will need a big turbo
I got told the hybrid V6 tdi is good for 300bhp, with that amount of power IMHO the only clutch I would look at is a Helix clutch rated to well over 400lb.ft. Exhaust, a full de-cat 3'0inch system should do once you have the custom needed manifold for either end. Software, take your pick, thats down to who you prefer, im sure a few good mappers in the UK could do it? Jabba? RS? P-Torque? As far as injectors go, id say you will need to upgrade, to which ones I dont know. Alot of people upgrade to say get 250hp however ive been told that the standard ones are good for this, anything after will need them. As far as what else is needed, id imagine it being very laggy even with 300hp so a NOS system for anti-lag perhaps to make it truely fast? Then obv you will need a L.S.D + everything handling mod available, no point having that power and not putting it down on the tarmac now is there. Ohh and W.I with a hint of methnol to keep temps down id imagine would be a good thing.
You will need alot of money