Rarity rating for that car on Mygranturismo.net is a 7, possibly because not many people bother to either keep one or buy one. (1 being common as air, 10 being elusive as rocking horse poop) But as others have already said, no car is truly rare, with the possible exceptions of the ones that you can only get from winning certain events.
Based on the MGT rarity rating system, the Sirion X4 is pretty rare! (rated 9)
My current 10's include Ford GT '02, Honda NSX '90 & '93, Honda S2000 Type V '01 (EU & US), Infiniti FX45 Concept, Mitsubishi CZ3 Tarmac Road Version, Mitsubishi i Concept, Nissan 370z GT Academy and one of the most common cars on British roads, the Vauxhall Vectra!!! (I have others but they're really not worth mentioning, as some of them are truly awful!)