Best L/D > 25?Basically without running the numbers, my expectation for required cruise L/D would be >25:1. And also the engine power would be such that for takeoff there is good power to weight ratio while in cruise the power is like 25% of total available. We can assume a fuel injected Rotax 912 is sufficiently powerful.
Then you are either flying slow or high. Autoreply and others know sailplane polars much better than me, but what I have seen of sailplane data they have their best L/D at a speed lower than your requirements. An Airbus may have a cruise L/D, but that is at high altitude.
And the Diamond DA-40?
Best sea level L/D of 14(?) at 80 knot? Or am I misinformed by just Googling for information?
The large problem I do see in your requirements are utility and ultimate fuel efficiency. An useable airplane that is time efficient and reasonable fuel efficient can be made, but I would be very happy with a cruise L/D of 10 at 100 knots or higher than 100 knots for such a plane.