I'd add another choice as well. Ask around amongst your Spanish neighbours. Buying from someone who is still living round the corner from you is quite smart as they aren't going to want to "sh*t on their own doorstep" as the saying goes. I went into the insurance broker in my village to get my house insurance in place, and asked him to put the word around that I wanted a small, old, cheap car that would be reasonably reliable, and when I go back next week I'm test driving a 10-year old Seat Ibiza and an 11-year old Fiat Punto. The owners of both still live in the village, plus on top of that, my insurance broker has promised that the people selling are good honest friends of his.
And just in case neither of those are any good, the Spanish families either side of me in my little street are keeping their ears to the ground, and one has a brother who might sell me his.
I would rather trust the Spanish villagers who would feel let down if I was sold a pup, rather than buy from a Brit going back to the UK, where you've got no come-back.
So my advice is just chat to your neighbours.