When we decided to buy a car in Spain, we needed
a) my Passport
b) my UK Drivers Licence
c) copy of Padron document as proof of address
d) copy of my NIE document.
We visited a few car dealers, found the car we wanted, paid 300 Euros as a test drive and reservation fee, (Non returnable but deductible from purchase price on completion)
The dealer then gave me a copy of the 'permiso de circulacion' to take to an Insurance company of my choice.
We arranged the insurance through a local broker who gave us a cover note. We then took the cover note back to the dealer along with the balance of a 10% cash deposit for the car.
The dealer then made all of the transfer of ownership arrangements and paid whatever taxes were due on the transfer.
The transfer took a couple of days; we then had a call from him to let us know that upon receipt of the balance of the purchase price the car was legally ours.
One year later we had a letter from the local town hall requesting renewal of the Spanish equivalent of Annual Road Tax; for our car it was 78 Euros.
Two years after purchase we had a letter from the ITV agency letting us know that the car was due its next two year test.
Ignore this at your peril, just days after the renewal date, if you have not been to the ITV centre your details are forwarded to both te local and National Trafficos.
Couple of things we have since learned; if you want fully comp insurance you have to ask for it otherwise you will get 3rd party fire and theft cover along with (generally) roadside recovery built in.
Fully comp insurance is not the same over here as it is in the UK, and you will need to study the small print very closely to find out exactly what is covered.
It is very doubtful that you will get the declared value or purchase price back in the case of a write off prang.
If you only have TPF&T insurance dont even contact your insurers unless you have all of the details of the second vehicle and its owner.
Always, but always carry the vehicle documentation in the car, including proof of payment for the road tax and insurance.
Ensure that you have Hi-Viz vests available for all occupants, spare bulbs (and the tools to change them), and if you need specs for driving, a second pair of glasses.
A couple of years ago during a particularly aggressive anti-speeding campaign along the CDS, the trafficos got a nice little earner by inviting offending drivers to leave their vehicle whilst some technicality was explained. Drivers who exited their cars without first putting on the vests were additionally charged with this offence too...
Hope all of this helps....