I second the idea of just buying the car seat you really want for your car and then buying a Scenera for traveling. This is what I am looking into right now. I will eventually use a Radian for traveling, but since I have yet to find anyone who has installed it rear-facing on a plane (and it sits very upright while FFing) and it seems it is a pain to install with a seatbelt, perhaps especially in foreign cars, I am thinking about using a Scenera for traveling with him while he is small.
The Scenera is often recommended for traveling because it is a cheap seat that is lightweight (at least 7 lbs lighter than the Radian) and is an easy seatbelt install on the plane and in most cars. At least that's what I've gleaned from lurking around these boards.
As far as the legal issue goes, I thinK Wendy is correct unless you are visiting family in Italy. I think if you are using the car of someone who lives there rather than a rental car you really should have an approved seat. But it sounds like you're planning on using a rental so I'm thinking this won't apply.