(Hello to the fellow 2CV owner. I like my W126/2CV combo. And ps Derek, DS or SM? You have excellent taste in cars, what with your old Mercs too.)
But on topic, sounds like you have the offer of a lovley car there C2raig, a weeks trial as you mentioned would be a really good idea. And if you have a garage in France, to do the occasional tinkering, the car may well do you proud.
I know exactly what you mean about the modern Citroens, for work i drive a Berlingo. Ugh! Its really not good at all. It would depress me if i had to drive it too much. Even the Seat Alhambra i drove previously was better than this.
My old W126 has done regular long journeys with seamless reliability for about two years now. Hasnt missed a beat. But i tinker with it a lot, adjusting things, replacing things i have found better cheaply secondhand but mainly trim or interior parts to get it up to scratch. This car has had a gap in maintenance that i have been catching up with, but it seems maybe not too big a gap, so its been the most reliable £405 i ever spent, the economy is bizarrely ok at higher speeds on a trip, plus when i drive it, it makes me veery happy indeed..
If you buy it and start to feel uncomfortable with the experience, there wouldnt be a shortage of buyers for one of these so i doubt you would lose out too much. (As long as no big nasties come your way maintenance wise.)