I second the C5.
Personally the 2.0 hdi VTR model after 2003 that has dual electronic temp control. VTR as it has a rear windscreen wiper as standard which the others, including the Exclusive didn't have. There are a couple out there with sub 70k for around £2500. Down side is that they don't have cruise control.
I wouldn't go with the facelift from 2005 as most are 1.6 hdi 110 which hasn't the torque of the 2.0 hdi although they came with cruise control. It also has the PAF exhaust filter which PSA didn't get the cleaning cycle software sorted until a few years later. i don't know if there is an ecu update. I wouldn't consider the 136hp model for the same reason. If there was a good example though, I would give Citroen technical a ring as check out if there are any software up grades.
My brother in law has an Alfa 159 on a 57 plate, and he has to clean his PAF filter out about every 6 months.
I would also consider a Mondeo Tdci around 2006 - the last of the old shape. The 130hp seems to be the most popular, but they do have injector, fuel pump and dual mass flywheel issues. They also have higher mileages with lower mileage example fetching a higher price. Personally I would stay away from the Titanium and 155 hp models - low profile tyres are expensive to replace.
Spruce