Road tax thing is correct info, but road tax is a provincial thing here, paid at your council office, (one you sign up as ‘Padron’ at), registration etc is at your provinces main office ( usually nearest main town/city), but provinces can differ in their systems...its not at all easy to navigate on your own, you really will find you get bounced from one office to the next and back again, if you have a ‘Gestoria’ near you that deals with vehicle registration etc.and they don't want a fortune for dealing with it its sooo much easier. My road tax is 35€ per annum,( went ahead and taxed it expecting no insurance problems, ironic as historic theres no tax fee per annum) multiply that by years off of the road will give you approx figure you may get hit for plus all the other fees/taxes, not extortionate amounts but due to the bouncing and messing about it adds up...
Bit of a warning, if your intending to re-register it for the road and are not going to register it as an ‘Historico’ ( UK =Classic) find out beforehand if you can get a normal insurance policy first, I’m having problems finding a company here in Spain that will insure mine as normal, I would have no problems getting ‘Historico’ insurance on it, but having already had and registered a vehicle as historic here I would not do it again due to the milage limitations imposed by the insurance (3000km a year, which passes with the blink of an eye, or a bit of nice scenery). My current 4 a 1985 F6, I had no problems with insurance as normal approx 18 months ago, different scenario now......its my only vehicle so I am not in the position to be able to say what the scenario would be if it was added to an existing policy as an additional vehicle. Best of Luck