My impression is that dwarf goats in Spain are more of a conversation piece than a practical addition to a project. From what I´ve read, Nigerians have to be carefully bred to be dairy producers, and even then you have to be careful of small teats and tiny orifices. I´ve never seen anything advertised over here with that purpose, so maybe they´re just lawn mowers.
Is there a regional rare breed you might try? Biodiversity and all that.
http://feagas.com/index.php/es/razas/especie-caprina#.VkUcfV55K8A
It´s still early days for us, so I can´t help on your practical livestock questions, sorry. I look from time to time on segundamano.es, just to see what´s on offer. Seems like importing live animals from elsewhere would be a paperwork nightmare.
Good luck!