MCKES,
Talk to Winrye Aviation at Bankstown, Brian Chadwick, about Merlin operating costs. They have rebuilt the two DCA Merlins, and put then back into service, (well, one and one about to go) plus have a lot of experience with them.
Both aircraft have very long legs.
The Merlin is a bit more roomy, but does not have anything like the field performance of the Commander.
Do a very thorough inspection around the nose wheel area of a Merlin, repairs to serious damage in the area is common, leaking fuel from the centre section is also a good indication of heavy landings.
Don't even think about either, unless they have been upgraded to at least -10 engines.
In my opinion, a Grand Renaissance Commander (effectively re-manufactured) is the best way to go, look up the US web sites for Twin Commander Corp., that will give you leads to the major sources.
Twin Commander Aircraft LLC
GAM are the Australian experts on Commanders.
Tootle pip!!