I have a 1.6 Focus, had it now for 3 years (as in sig pic below) not had any really major issues, just a couple of small niggles, that were easy and cheap to sort out.
As long as what your buying has a full service history and well maintained you should have no massive/major problems

A 1.6 will be a better compromise for economy than a 1.8 or 2.0 for the type of driving that you're going to be doing.

The 1.8 is not that different to the 1.6, the only difference with the 1.8 is it has better mid range grunt.

OK a 1.6 wont be a rocket ship compared to a 2.0 ST170, but what is it that you want, a flying machine or a decent run around that'll be cheap to run?