I echo the sentiment of buying a Japanese Duo R. They are usually cheaper than the US unit, and oh so sexy. Japanese hucards are generally cheaper as well, and there aren't too many US released hucards that are unplayable in Japanese.
You don't need a different power adapter, Japan uses the same kind of power as the US.
However, be warned, your wallet is going to take a beating. Once you get the system, you are going to want to get:
New controller
6 button controller
Multitap (even if you don't have any friends)
Arcade Card
Tennoko (sp?) bank
PC Engine GT
Backup system, just in case
Flash cart
A friend in japan who can hook you up with cheap games
You have been warned
Haha. Don't scare him!!
But a lot of those things can be had for pretty cheap. 6 button controllers (i.e. Avenue Pad 6), and a brand new boxed DuoTap can be had for $10 each. The Arcade Card is also a must, but those can go for pretty cheap as well. I would suggest getting your Duo modded as soon as possible, just to get it done and over with.
A backup system is also not a bad idea. I have a TG-16 as a back-up, but that leaves most of my collection unplayable. hehe. I foresee myself getting a Supergrafx w/ the SuperCD combo unit. The day that happens will be a sweet day.
Anyway, don't worry. Your wallet will survive the turbo madness.