Having had my Roadster since about one year, and 12000km, my average consumption settled at around 9 l/100km.
As a V6 owner I have the same consuption. This engine is realy economic, and it's a pleasur to do 500km without refueling.
Perro, my advice would be to take the "bigger" you can afford.
The V6 is, well, enjoyable, really

And regarding the motorway, even if it's not the best place for a classic mog', the start after the toll is a great pleasure, which Mrs H3 enjoy the most.
(Mrs H3: "
Oh, darling, I know there's a speed limit! But there's no time limit for 0 to 130 km/h...")
H3