TAS661, how much money do you have to spend? If you are independently wealthy, then follow dragchute's good advice and get a new turbofan aircraft with factory training, warranty and support. It will save you a lot of headaches!
If you are on a budget there are two tried and proven types which satisfy your payload/range criteria.
For under a half million dollars you can't go past the PA31 Navajo Chieftain. It is slightly faster and an all around better load carrier than a Cessna 402A,B, or C.
For three quarters to a million dollars, you should look seriously at the Cessna Conquest II. This aircraft will carry the same load as a Beech Super Kingair. It is cheaper to buy, has lower DOC's and has a better range and turn of speed than the KingAir.
If you want to know about Conquest II's, talk to Bob Douglas of Douglas aircraft sales. He sold them for many years through Rossair in Adelaide. Last I heard he had relocated to the Gold Coast. Look in Trade-A-Plane for his advertising and contact details.
The Oracle!!