While some people may like to consider rarity by how often a car may show up in the UCD, I refer to the car list at mygrandturismo.net. That site sorts the cars by how frequent they appear in users' garages. In this case, the M Coupe is a level 4, which I wouldn't consider rare, but more like undesirable. It seems to be a car that people collect simply to own the model.
Bingo. It may not be a bad car, and may be underappreciated, but it's not widely accepted. I, however, wasn't using UCD appearances to gauge rarity, only to show that it's fairly easily obtained from there and its uncommonness can't be blamed on that.