The 2ltr also uses a different gearbox, and as far as I'm aware, you can't just use the Ib5 that's in already, unless you put the smaller flywheel on from the 1.8, plus other issues, probably.
Starting with an 2ltr at least, sound's like a better option. as others have said. I wish I could have got a 2ltr, but I couldn't get a 1.4 for the price I got my 1.8 for.
Then again, you could still get the 2ltr engine and rebuild it to put in your next focus.