Going by my own car importing experiences. Keep the bike to manufacturers spec. Any extras added and the Spanish get confused. Bring all the standard bits with you, bung them on for the ITV, swap them back to your own bits after.
If you look in the free papers, there are plenty of companies that offer import services and its worth using their services to relieve your own stress.
Any good importer should be able to find out the taxes you will pay plus give some idea of the total costs excluding what needs to be done for the ITV. They will need a few days to find out how much the taxes you pay will be.
Road tax can vary from area to area, since its not the same price across Spain.
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