Another thing to bear in mind is what your input AC voltage actually runs at as the stepdown transformer will only step down the voltage at a set ratio of the input. Before you buy a step down it would be worth carefully measuring your mains voltage to see what it's running at.
When I bought my last Astro City it came with a Maplins 230>110v step down transformer. I measured the output of the transformer and it was outputting 120v which is far too high for the chassis.
Upon inspection it seems that my mains voltage runs at 245v.
I'm in the UK and the nominal voltage here is 230 Volts, however by law there is a tolerance of +10% / –6%, which means anything within the range 253 Volts to 216.2 Volts is 'acceptable'.
Even an Airlink 230>100v step down would have given me an output of ~107V which although is closer to 100V it's still a little higher than I would like.
I ended up using a SKOT 560-5422UK REV 1 which is one of the standard stepdown transformers found in a UK Naomi Universal. Here's one pictured below when I was messing around with one:
The beauty of these 100V step downs is that they have a configurable input and you can select between 210V, 220V, 230V or 240V in. I ended up installing this in my E2 so I can run the cab directly from the mains. As it's a Naomi Uni PSU it also has a 17VAC output used for the Naomi audio amplifier, bonus! My E2 didn't have a audio amp so I installed one at the same time.
Before:
After:
I cut a new larger board for the base and mounted the original parts to it along with a few extras. I've got the original board still so it could be reverted back to stock in minutes.
Since I had a ~230v input I took the opportunity to add a couple of internal sockets for consoles etc and mounted a ~100VAC voltmeter to keep an eye on the output. As you can see using the 240V input on the transformer gives me a nice 102VAC output
So yes, it's a good idea to check your mains output before committing to a stepdown. As well as running too high you can also have problems if your AC is too low. In that situation you may want to use something like a ~230>110V stepdown rather than a ~230>100V