If you're serious about doing it, have a drive of my car at some point (International/Snetterton maybe?). It's on really, really hard springs and will be a lot lower than it is in the last pictures I posted of it, but it'll give you some idea of what the car will be like. Harrys car (the red/yellow Collins plant hire one) is much softer than mine but that's no bad thing as it's far more confortable in use, and it handles really nicely. It depends how far you want to go, an inch or two, as Joolz says, is easy and you don't need to modify anything, but if you want it scraping (I don't see any point in doing it any other way) then you're looking at spending a few quid and changing stuff, and it will affect practicality to a degree.
Of course, you could go the whole hog and channel it, as Roy suggests, but you're looking at doing a lot of work (you basically cut all of the internal structure of the car out and remake it) and it'll be a cock to return to standard again (you'd have to redo all the work you had to do to channel it, and that's potentially a bigger job). This is only worth doing if the sole aim of the car is having the body as low as possible, for purely aesthetic reasons. you're still going to have to do the same work to the suspension as you would simply lowering it.
I'm not sure I like channeled cars, they can look 'wrong', in the same way the average Sidewinder conversion can. A 2cv is a very difficult vehicle to modify the appearance of without making it look strange.
Obviously you won't be looking at the car from this angle, but the absence of anything protruding under the bodywork makes it look a bit odd in my eyes, but from this angle...
...it looks like a normal 2cv, which is nicer to me.