Also check out the Citroen C5, uses the same range of engines as the 406. The New C5 2.0 HDi is somewere in the 130 bhp range and is very good on everyday economy 45 mpg. MIne is the earlier version which is 2.0 HDi but only 110 bhp.45mpg is no problem more if your light footed but this drops to around 32 when towing. I tow at 87%, Bailey Ranger 510/4. This would be less on the New ones as the kerb weight is 1500 ish with 75Kg at the tow ball but only 1400kg max tow, 1500kg for the earlier versions. The hydrolastic suspension works well and you dont need to uprate springs etc. From memory when I was looking at the 406 and the C5, I think the C5 has the edge for room and comfort. Both have won tow car of the year albight the 2.2 version of the C5.
Hope that helps