There is a used car dealer near San javier airport called SVG. There are also a lot of main dealers who have 2nd hand in Cartagena. They are along the Los Dolores road. They usually have an english speaker on staff. We bought my trusty Twingo from the Renault dealer and had excellent customer service.
Our last car came from Torrevieja area where there are loads of second hand car dealers and usually speak english but aftercare not as goood as the Spanish Renault dealer and we had spent 4 times as much .
We took some friends up to Totrrevieja for a look along the N332 strip and they bought from Pilar de Horadada at a little garage where they sell tyres just off the N332. The girl in the office speaks english but they only have 1 or 2 cars.
Most of the second hand dealers advertise in the free press so you can get some idea of prices. They are more expensive than UK but would not want to drive a right hand drive over here and you have to factor in the cost of the transfer to Spanish plates.
Jean