Welcome to the forums from a fellow european.
It all boils down to either getting one that already is in Spain, or actually importing one. If it's your first one, I'd recommend trying to get one that already is in Spain, so you can testdrive it prior to buying it to see if it works for you or not.
Your best bet would then be to either contact the
Spanish Mustang club, and go through car dealers to find that Mustang that you want. Model, age, engine, price, whatever you feel you want.
Based on that you can locate various notices about the Mustang, and find out known issues with the car, so you can pay specific attention to those issues prior to purchasing the car.
Be aware... actually importing the car will up the price of the vehicle. Be very sure you are aware of the costs. If possible locate someone that imports cars from the US or has expirience in doing so, so you are sure to handle taxes, shipping, and whatnot in a proper fashion.
Also know that parts and maintenance for the Mustang are a bit harder to come by. I know my Ford dealer (who I asked prior to purchase if he could support the car's maintenance) needs about 2 to 4 weeks to order parts if something breaks, and I try to take that into account prior to it's yearly checkup.
Also inform yourself of insurance and any taxes that come with the car. It's a shame if you buy the car, then find out it's too expensive to actually drive.
Other then that... have a read at these:
http://www.allfordmustangs.com/forum...mformatey.html http://www.allfordmustangs.com/forum...rom-spain.html http://www.allfordmustangs.com/forum...ologation.html http://www.allfordmustangs.com/forum...ing-spain.html http://www.allfordmustangs.com/forum...t-get-old.html http://www.allfordmustangs.com/forum...-in-spain.html http://www.allfordmustangs.com/forum...rom-spain.html
Hope that helps you a bit in making your choice, and the possibilities that exist. Let us know how you get on
help buying a mustang in spain 
