I will admit to not generally being a fan of Mercedes-Benz but were I a billionaire and had a car collection, I would give serious consideration to adding a 1955-58 Mercedes-Benz 300Sc cabriolet or roadster to my collection. The 300Sc was also available as a coupe. Just 200 cars -- of all three body types -- were made during the production run. I would think a 300Sc would compete well with a Rolls, but with improved driving dynamics. And the 300Sc just exudes old world charm.
3-liter straight six with 175 hp.
Also, it would pair well with my (imaginary) 300SL roadster of slightly later vintage.