You can transport argon (or mixed) bottles laying in your back seat. I got given the all clear when I hired a bottle from supagas.
As for how much gas is needed all depends on how much rust there is and whether you are welding outdoors (more gas).
I had the usual rust in dog legs, tail light region, hatch, fenders, doors, roof, and I managed to repair it all with an E size bottle. It included practice, Coilovers conversions and other random jobs. More than enough.