I really was too excited and was not thinking when I was shopping online and ended up importing a Japanese Vox lil Night Train into Australia. Japan's voltage is 100V, whereas downunder 240V. I'm thinking about getting a step down transformer to run the amp. Since it said 30W usage, I'm guessing a 50W step down transformer should be ok? Would running a guitar amp through a transformer be any problem at all?

Many thanks for your help!!!
That setup will work just fine
There are quite a few people here that use step-downs with amps all the time.
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
Most major bands that tour Australia have to use transformers (usually hired for the duration of the tour) if they have bought their own gear over. Should be no problem at all, just make sure it's a decent transformer as cheaper ones tend to result in unwanted noise.
It's an opinion. It's subjective. And I'm right, anyway.
I've aactually been running my amps (and a lot of rack units for that matter) on a step down transformer since all of my stuff is from the US but i'm mostly in Europe now where it's 240v. Currently in Armenia, and I have been running a project studio here on stepdowns. hehe.

Tell you what, if you mix 110v with 240v gear even (I mean plug say a rack that's on 110v+stepdown to a 240v rack or amp) won't get you any issues.