Does this do what you need:
contestoDB.DataBaseCar.Where(x => x.Class_ID == 1 &&
x.idMarca == "6" &&
x.nomeModello == "2-Serie" &&
x.specie == "CAR")
.GroupBy(x => new { x.Class_ID,
x.idMarca,
x.numModello,
x.porte,
x.body,
x.nomeBody,
x.modelloDa,
x.modelloA,
x.specie })
.Select(x => x.OrderBy(y => y.nomeFileVeicolo))
.Select(x => x.First())
.OrderByDescending(x => x.modelloDa).ToList();
There is no need to perform a distinct since you have already used group by for all the columns in select.