Ok, the Mk2.5 (08-11) has three options I think for a diesel engine. Has the 1.6 litre TDCi, the 1.8 and I think a 2.0. The 1.6 Litre however isn't a Ford or Duratec unit, its actually a Peugot unit. Its quite notorious for blowing and munching turbos. Something about the oil lines getting cooked, anyway, they come up a lot on ebay and that with needing turbos replaced. That said, if the service history has been kept up to date they should be ok.
I think since the 07 plate they started using the DPF in the exhaust system, early ones used a different type to the later DPFs that require the fluid to be topped up.
Not too many horror stories on the DPF front as long as you occasionally give it a good thrashing to burn off the filter. I must admit I don't know the full ins and outs, but the service for the fluid to top up (which has to be done) is about £300-400 apparently.
If you were looking at a modern focus like mine, I can't recommend the 1.8 petrol enough. I must admit I'm disappointed with the fuel economy at the moment, averaging about 30mpg. But, some big pluses, its a Yamaha unit, it has cam chains, quiet and really refined. And I know its not your big yank lump with a bellowing V8 but it does make me smile when you get around the 4k region.