You cant get a cheap second hand car in Spain as they don't depreciate much. I went to a reputable dealer who gave me 1 years warranty on the car and did all the transfer work into the price. It's a 2007 car and was 4 times more expensive than in the UK but I am happy with it and having the warranty gives peace of mind
We have a detailed guide on the process of purchasing a car that should help you get started: Buying a Car in Spain
Hope this helps!
M