Recommend: PC 12 or B200 - both have similar size cabins and can do it all - dirt strips to major airports. Just look at the companies that use these machines - MAF use PC12 in Sudan, UNDP use the King Air in Similar role. Most Flight inspection / Calibration aircraft wordlwide are King Airs - they are very well built and twin P&W engines are super reliable. B200 will attain F350 but beware of front screen cracking and water in fuel ice issues!
I think B200 operating cost is around 800 GBP per hour (excluding crew costs) - there are some 6000 examples worldwide so spares is no problem. C441 is faster with better range but less reliable and more cramped!