Thanks a bunch for the enlightening comments.
The C T303 Crusader definively looks like a candidate I've overlooked so far.
As for the airstrips planned: Part of the ops would be Botswana and around, that is elev. 3000ft@25-35C, so pretty hot&high. The strips, although called "Bushstrips" are often well maintained (e.g. "Calcrete", i.e. natural hard surface from dried mineralized water), but the occasional bump or stone should not destroy the way-too-fragile U/C.
The 1000 NM range are probably the upper limit, 800 NM may do equally well.