Hi
I have just got my car back today, it has been changed to Spanish plates, it cost 804 euros, and this did not include the headlights! I had them changed from RHD to LHD for the ITV (Spanish MOT)
My headlights cost 60 Euros from a scrapyard but beware if you have an unusual car like a Kia you will find it very difficult to get headlights from a scrapyard, and the cost of buying new is horrendous ( look on the internet first for LHD headlights this a major cost when putting a car on Spanish plates.
As for tax I sorned my car in the UK, but kept my insurance up to date (you can get special insurance in the UK that lasts up to six months travel abroad) you can also get Spanish insurance on English plated cars through a company like AXA Spain, Linea Line, or Abbey gate insurance. I would suggest you bring the MOT certificate over with you as well as the log book, depending where you are going in Spain depends on how lenient the police are.
At the moment Spain is in a recession and the police seem to be targeting English plated vehicles (I can’t be sure but I think that if you have a car in Spain for over six months you are supposed to get it changed to Spanish plates.
I found cost effective and in the long run just to put it on Spanish plates, but in certain circumstances it would pay you to sell your car in the UK, and buy Spanish here, but again you need to have NIE numbers (which I got at Spanish embassy in Edinburgh) and Padron, (Which I got at my local town hall, I only had to show my passport and a document to show I was living in Spain, without this Padron you cannot buy a car legally)
Please beware! The cost of cars here is stupid try a way up the pros and cons
If you’re coming down Costa del Sol I got a person to do all the paper work for me.