How can anybody recommend a petrol or diesel to you without knowing how you will be using it? You didn't even specify how old or what Mk. of Focus you are looking at, although at 3 or 4 grand it will be a Mk.2
I have the 1.6 petrol (100ps I think). I've had it since May with no problems so far(shouldn't be, only 43k miles) but boy is it slow. I mainly use it for 120 mile journey's down the motorway, and get just below 40mpg.