good plan. i hope you find something cheap.
the earlier you book the cheaper it is, leave comments below to look for carshares.
here are some websites :
http://www.easyterra.com these guys have cheap deals from Reus airport. 50 euros for 3 days when i looked just now.
http://www.pepecar.com and pepecar are always cheap from BCN but check the limited km’s
don’t forget a print out of the booking, your driving licence and a credit card