Hi Michael,

Having contacted you by e-mail since we have a better idea of your budget and we know that you are trading in a 2006 Opel Vectra 1.6-litre petrol with 40k on the clock. You could get offered around €7,500 for your Vectra (depending on condition) and add your budget of €6,000 to this, so you'll have €13,500 or so to spend. 

Of the cars that you mentioned the Peugeot 407 will no doubt offer the best value for money, quite easily in fact, with something like this 2007 model likely to end up within your budget. The 407 diesel is great for economy. The quality isn't bad either. You might get the odd electrical fault, but by and large these cars are capable of quite high mileage. I'd be more inclined to spend a little less if I was buying one. 

The other cars you mentioned are the VW Passat and Mazda6. There are lots of Passats about so this means that there is lots of choice and the prices are pretty competitive too. A diesel one would put up with high mileage pretty well, like this example, which would be a nice car, feeling more upmarket than the Peugeot. They aren't without their faults though and the Passat has had issues with the handbrake and some other items.

Lastly is the Mazda6, which is a grossly under rated car and you could have a fully loaded 2.0-litre diesel version with low mileage and loaded with kit for €13,950. Of all these I would probably buy the Mazda, as it should be ultra reliable and there aren't too many reported faults with it. 

I hope this helps, if there are any other questions then do come back to us.