If you go for a mondeo get a petrol ghia X, the mondeo diesel is old and rubbishy.
If you go for a 406 get an HDI diesel, the 406 petrols are mediocre, they only make sense if you get a true bargain and drive very few miles a year (like me)
I have a 406 2.0 petrol executive. mpg is 33-34, the car is no faster than my old 1.6 306 at low revs, but picks up higher up the rev range. It needs working quite hard to get good performance out of it, but it has a lovely ride and is great on the motorway. 40mpg is *just* possible, with the cruise set to 70.
Mondeo has a better petrol engine, but the 406 is better looking (imo) and is better specified.
Let me know if you want to know anything about 406's.
Other cars to consider:
Passat? not a great handler but high quality inside.
156? Questionable reliability, poorly specced but looks the part and is cheap.