Hi Iain --
There are two taxes you pay on cars here in Spain. One at the time of purchase and an annual tax.
The fee that you pay at the time of purchase depends on if you are buying your used car from an individual or from a company/dealer.
If you buy your car from an individual, the tax is called "impuesto de transmisiones patrimonales" (essentially a property transfer tax) and if I am not mistaken, is based on the value of the car.
If you buy your used car from a dealer or other business, you must pay the VAT which for used cars is 16%.
It also depends on how that car is registered. For example when we bought our Peugeot (used) last summer, we bought it from a company, but it was registered to an individual so we paid the first tax not the VAT.
The annual tax you pay is definitely based on the value of your car (something the State determines). I suppose this could be compared to the annual registration you have to pay in the USA (at least in Pennsylvania) for your car. Although that annual fee was only about 45 dollars.
In addition to the annual tax, you have to pass inspection (ITV) every year if your car is older than 10 years. Under 10 yrs old, you have to pass the ITV every 2 years and older than 20 years you have to pass it every 6 months.
ITV is a government controlled activity and you must go to an ITV center, not like in the States (or at least Pennsylvania) where you can go to "State Approved Inspection Stations" --> gas stations with garage/mechanics.
If you plan to do a lot of driving here, I recommend that you buy a diesel car rather than gasoline. Gasoline is very expensive here. Our Peugeot (it�s a large car, the Peugeot 605) is gasoline powered and costs 70-75 dollars everytime we want to fill the tank. As you can imagine, we don�t drive it too frequently. Incidently, it�s in the garage getting prepared to pass the ITV.
Hope this info is helpful to you!