The first thing I would ask is how many miles are you doing each year ?

As said if you don't do many miles then go for a petrol and I would suggest the 1.8, the 1.4 will feel under powered and the 1.6 may not give you many more miles to the gallon than a 1.8. As said make sure the cam belt has been done on anything over 8 years old or with more than 80k on it (my mate works on Focus's and Ford advised them of the lower interval instead of 10 years or 100k !). Check under the door mirrors as they can rust where the mirror starts to rub through the paint on the door skin. If you get a 5 door have a really good look around the edges of the doors as they can rust quite badly if left unchecked, they can also suffer from leaks from the thermostat housing but its by no means a big deal. Apart from that its a case of all the checks you would carry out on any used car but on the whole they are pretty honest motors.

I have a 04 plate TDCI Sport that I bought on the recommendation of my mate and after almost four years of ownership I still love it. It goes well enough, is good on fuel and with the 17's on it the handling is fantastic !