Taxi Licence from local council and depends upon availability. Normally not issued new and you have to buy from someone prepared to sell . I took 18 months trying to find a solution and had to seek advice fom lawyers and paid a gestoria to sort out minibus.
No its not easy. You have to be autonimo ( self employed) for a period of time prior. I became autonimo with months of getting here and also you have to have insurance on vehicle that covers clients. . Normal car insurance is not good enough. Vehicle should be Spanish registerred with correct and valid ITV badge on windscreen to show it is road worthy. Any legal business in Spain should have cjhanged plates to Spanish
I was informed that Excursions ie acting as a Tour Guide is different . Ensure you have client insurance ( the amount that can be claimed is far higher than ordinary car insurance) and you are registerred in Tax office for that purpose.
David