There is no commonsense when Government's buy or lease aircraft; none at all.
The G550's are expensive with the equipment, but my understanding this also covers 10 year life/maintenance support and maintenance included in package. Maybe even engineers as well?
Dassault 7X to replace CL604 is a very good choice. The 7X by reading respected journals have far superior short field qualities than Global, Gulfstream products. It also give RAAFies a different 'stick' to play with, experience is good. Better still they will be able to do a 2 engine ferry for maintenance if required. The F900's prior to the Challengers were apparently problem free...