Hi Jil.
Mrs welshwizard and I had thought about driving to CdA.
We have both driven the length and breadth of the UK over the last few years doing our jobs and thought "how difficult could it be"?
When we looked at the map of Europe and calculated the distance and driving times, we decided it might not be such a good idea after all. I think it will probably need at least 1 overnight stop and depending on the fuel tank range of your car, maybe 3 fuel stops. You have to consider the cost/hassle of catching a ferry to Santander and driving accross Spain or hopping on a cross channel ferry and driving for 17 - 20hrs down through France/Spain. Only you guys will know how much driving you can put up with. Don't forget to factor in European road Toll charges when calculating your costs.
Good luck what ever you decide.
WW