id go standard 2L, yeah its not as easy to install as the 1.8 and itll cost more, but youll get more torque with it and a bit more power and the potential for more

uprate other things first though as Gingerturbo says, brakes then suspension, then look at fitting the gearbox if you feel you want that

then fit then engine and make sure everything is good with that before you tweak it. id add, exhaust, airfilter, cams, vernier pulleys then look into head work.

it all depends how far you want to go and what you ultimatly want. theres no point in fitting the zetec and tuning that if what you really want is rst power