I would suggest booking a rental car online or through a reservations agent, just to be sure you will get the size and features you would like (you can probably even do this once you arrive in Spain, if you find an internet cafe--just check some prices and locations online beforehand so that you know which agency to use). I have sometimes found that, with things like car rentals, you are at the mercy of the agent if you show up with your tons of luggage and a desperate look on your faces! Then you're told that the only car available is a super-size Mercedes for $1000 a week or something like that!!
I have also found that accomodations are quite plentiful at that time of year, though--as long as you're not coming into town just as a festival is taking place.
I don't know about trains and buses at that time of year--but someone on the board is certain to have lots of info about those!