I've been in the BBJ a few times in my previous life. The aircraft, then anyway, were very basic.
If you get the impression these aircraft had gold plated taps and spa baths think again. They were meant to be airborne offices and deliberately understated to keep the voter happy.
I agree with Keg. We send these aircraft overseas with the leaders of our country aboard. Regardless of your politics, I'd prefer to see that service provided by the RAAF in an aircraft designed for the purpose. I'd love to see the RAAF doing troop transports too rather than via the lowest bidder, but that's another issue.
Not only does a dedicated aircraft allow the ministers the opportunity to work in transit, it also gives them a certain status that commercial flights don't allow. In the wider world of politics, this status is actually worth something.
I reckon B789s would fit the bill nicely.