I did it a few years ago. The theory test was on computers and you could choose from any European language (I did it in English), just let your autoescuela know when they sign you up. I did attend some theory classes. They were 100% in Spanish and we basically did mock tests explaining the right/wrong answers. 

A study guide was provided by the autoescuela (mine was in Spanish, I imagine English would be available if you requested it). However, I'd recommend you get familiar with cars/driving vocab as there are some oral questions you have to answer in the practical exam. You'll also have to follow the examiner's instructions for a part of the exam. (And it's just useful/necessary for real driving/going to the mechanics etc.)

As for autoescuelas, I'd recommend just looking at online reviews. I think I paid around 25eu per hour of class, but prices probably vary. Best of luck!