Ford Focus have been around a very long time & keep selling well.. from what I can see they are a good solid reliable car... I have ford diesel myself (not a focus) and have had no issues with it... If the car "fits" - then go for it... you should be entitled to the 2 year warranty (think the 2012 models come with 4 year). Haggle on the price, get extras thrown in if you can... cash is still king at the moment...
One thing with the diesel (across all new cars - not specifically Ford) - is the car going to be driven a lot or just city driving? Why I ask is that I saw a programme on BBC (not too sure if it was "watchdog") that highlighted issues with diesel cars only been driven around city & the filter thats in place to reduce emmisions becoming clogged up. Solution initially was to take it onto a motorway once a week to get up the revs on the car for 30+ mins... but over time that may need to be replaced if not looked after...
Been driving a 1.6tdci Zetec (95PS model) for last 6 months, have 9000km on it and no issues to date (other than a recall for a software update that took 20 minutes). Very economical especially when compared to my previous 1.4 litre petrol. I do mainly city driving but have taken it on a few long runs with a full load and it was more than powerful enough. If you want to know anything specific just ask
It's a 95PS in the current model and I'm more than happy with the power. I've done a few long runs with full load and had no problems overtaking, etc. I'm afraid I don't know the answer to the dpf question