ISV = Imposto Sobre Veículos (Tax On Vehicles)
I have been looking into this extensively, as we are about to move to Portugal. The tax is steep, and you are NOT exempt coming from another EU country, unless you have owned the vehicle for at least 12 months prior to transferring residency to Portugal, and have had a valid drivers license for at least those 12 month as well. The English website for the consulate in Washington (I am American) specifically states that the exemption to the ISV cannot be used unless coming from another EU country, or a Portuguese citizen that has been residing abroad for 12 or 24 months (depending on reasons for foreign residency). However, neither the website for the Autoridade Tributário e Aduaneira (Tributary and Customs Authority), nor the law itself (Lei n.º 22-A/2007, de 29/6, artigos 58.º a 61.º) seem to exclude me. In fact, both indeed mention that a person transferring residence from another EU country, or a third country, can benefit from the exemption.
The exemption can only be used once per ten years, and there are restrictions on who can use the vehicle. You cannot even lend it to a friend for the first year after importing it. Only a spouse or other family can use it. I also read somewhere that you cannot sell the vehicle for five years, but I cannot find that reference again.
My thoughts -
If you are coming from another EU country with license plates that don't stand out, you can probably get away with not transferring title. Considering my vehicle will have to arrive on a boat, and has Florida plates on it, I will likely not be able to "fly below the radar."
If you prefer do things properly (always a good idea), do NOT buy a car in your country of origin just prior to moving to Portugal, unless you are prepared to pay the import duties - because you'll have to. I also don't think that car prices vary enough from one country to another within Europe to justify the hassle. If you need a car, and don't already have one (for 12 months), it may just be easier to buy it once you arrive.
I have some useful links for reference, but could not post them because I have not made at least five posts yet... Email me if you would like them. smbackman @ yahoo . com. Two are in Portuguese.