Personally I'd save all the hassle and get a Cupra, I love doing the mods myself, but what you are trying to replicate already exists. If he got it cheap then he could potentially make a profit should he sell it thus giving him more money to buy a Cupra.
Other things that may need to be considered are the braking system not just disc sizes and calipers but the master cylinder, servo and if you need a feed for a hydraulic clutch (1.4 may have a cable clutch). Rear brakes should also be considered.
All in all the amount of money needed to turn it into a 1.8t would be silly