Hello, I hope the following assists you.As long as you have a European certificate of conformity then there is no problem registering a RHD car in Spain. As far as I'm aware it's not possible to register a RHD commercial vehicle in Spain as it doesn't meet the ITV standard due to a visibility issues.
Real Decreto 2822/1998 Articulo 111. Deben estar construidos y mantenidos de formaque el campo de visión del conductor hacia delante,hacia la derecha y hacia la izquierda le permita una visibilidaddiáfana sobre toda la vía por la que circule.
You can check with the ITV people but there are so many posts on the internet about this. If you want an EU certificate of conformity try https://www.eurococ.eu/en/certificate_of_conformity