It is kind of hard to say what to take, the weather has been so wierd the last few years. I suggest you prepare for just about anything, as the terrain and latitude and altitude of Spain varies so much. You will probably have BEAUTIFUL weather on the Costa, maybe some rain, but Madrid is another beast entirely, and can be rainy, windy, cold. . . or warm and nice. . . on alternate days! If you plan on going to the Pais Vasco in el norte, you will encounter mountains and maybe some snow. Probably cold and wet. I say take the kitchen sink, or purchase it here.
Best in paint,
Martín de Madrid
President, 1st Class Pinups
"The Classical Alternative in Pinup Art"
http://www.1stclasspinups.com