Sounds like that guy is concerned for the welfare of his fish, which bodes well for anyone buying from him.
I absolutely think you are taking the correct course by holding off. Frankly, if I were on the cusp of leaving for three weeks of vacation, there is only one thing that I would even consider changing right before I left: water! No new fish, or moving around old fish, or alterations to filtration, or anything else really. Stuff like that, IMHO, demands careful monitoring and observation for several weeks at least.
And...dare I mention the "Q" word...? I won't bring anything home from anywhere without a strict quarantine. The bigger and more established your tank is, the more you are risking by dropping in a new fish, even one from a promising and apparently healthy source like this one.
Edited to add: In thinking about it, if I were in this position I might offer the guy a deposit to hold the fish until I got back from Oz. Then you could unhurriedly set up a QT for it, while selling or perhaps filleting your mobas (again, nice fish...but only cichlids...) to open up the big tank for the Fire Eel.

If you don't have a biologically mature QT already, you could install a big sponge filter in one of your existing tanks before you leave, so that by the time you return it will likely be ready to install in a freshly-set-up QT.
Then you could start searching for another couple of Fire Eels to create that spectacular display. And while I have never had Clown Loaches, I suspect the two species would do very well together and would look great.
