I'm not a dealer but the most reliable car I've owned in the last 12 years or so (aside from the current Audi A4 Avant 3.0TDI Quattro) was a Ford Focus 1.8TDCi. I used this for my daily commute and had a Range Rover for other uses. Unfortunately, the Focus was written off at 98k a few years ago but in that time the only thing to go wrong was a rear window mechanism (easily fixed for £11 with a kit purchased online rather than the £400 a Ford main dealer wanted to replace the whole mechanism). I had the ECU remapped by Angel Tuning and it was a quick small car ideal for commuting with amazing mpg - I think I averaged around 58mpg from memory. I'd have a look at main dealer used, look at 1.8 or the current 2.0 diesel options and keep the options sensible.