If you pick up a car in Barcelona, you don't necessarily have to return there. You can return elsewhere in Spain. Some rental companies like Europcar (and I think Hertz) don't assess a one-way fee as long as you drop off within the same country. in the last year or two, I have used Europcar within Spain, France, Italy and Germany with different drop-off points. No extra charges were added.
But if you pick up in Spain and drop off in France, you could pay a few hundred Euros extra.
Also haven't driven it, but know some of the areas. I think you have two options - the autoroute via the Narbonne-Toulouse axis (A61-A64), or through the mountains via Vielha. Both are a long way - Google gives a minimum of 5h30 one way. The autoroute will be longer, but is pretty comfortable and easy driving and getting around Toulouse isn't too bad. Carcassone is an obvious point of interest about half way.
On the other hand, driving through the mountains will be slower because there are no dual-carriageways or faster roads and you'll pass through towns and villages, and have windier roads which makes for more tiring driving. However, it will be more scenic. I think you should be OK for snow if you're just driving through, but keep an eye on the weather forecasts as March is still the winter season. For this type of journey, I'd add a couple of hours for a stop somewhere mid-way which would give you about a 7-8 hour day of travelling.