You will also need a different beam and driveshafts!
There is alot of difference between the Cosworth and lesser models so take this into consideration before you even attempt to make a start, plus remember you will NEED to inform your insurer of the changes and hence a increase in your premium, else your policy is invalid!
This is why it's easier just to buy one outright, as in the long run it's probably cheaper!
You are looking at stage 3 power, so T34 turbo, 3 BAR MAP sensor, 803 Bosch green injectors or Siemens equivilants, this is plenty enough for road use!
If wanting around 280HP (Stage 1) then you simply need a remap!
Martin