1.9 is the older PD version and is reliable. 2.0 has more power and the BKD versions are more reliable, about year 2006 onwards. With all the TDI's drive them and listen out for any undue noises or excessive smoke which could be turbo on it's way out. Other than that, treat it as any other car and make sure everything works, drives and stops straight etc.
My opinion now is a petrol version will be less hassle and no longer more expensive to run, unless you are doing taxi type mileage. DERV cars can be mega expensive when they do go wrong these days.