Do you mean a newer Focus model? If that is the case, then you will find the Mk2.5 model which was not built after late 2010 early 2011 is basically the same car as the Mk2 under it's skin.
If you go for the Mk3, it is claimed the driving experence is not as good as the Mk2.5. that said, buying petrol you now have the option of the newer ecoboost engines. Better fuel economay, but not what is promised in some cases so best visit http://www.honestjohn.co.uk/realmpg/ford Lower road tax, but a big nono for me was the smaller boot.
And Ford have obviously not been totally happy with the Mk3, because they are bringing out the Mk3.5 in July. It's been said it is a marked improvement driving experiance wise. In fact a Ford designer involved in it's developement reckoned it should actually be seen as the Mk4.
You seem to buy your motors like I do, not new but paying circa 50% less than their list price when new. Infact this week I traded in my Mk2 55 plate Zetec Climate , against a Mk2.5 11 plate Titanium. Thinking being I will skip the Mk3 offering , and possibly buy a Mk3.5 if I can then live with a smaller boot.