Nokia have a fairly standardized set of PSUs - besides legacy PSUs for outdated products, there are only two or three current basic models, all 100-240V, with the usual set of local plug sub-variations. Electrically, you'd be fine with the US model - but the US localized plug would mean using some ugly adapter, and a travel converter set would be barely cheaper than a Nokia PSU. The neccessary standard Nokia PSUs are available at every cell phone store, for about 5-15EUR depending on whether you hit a cheap or expensive store, and whether you want the normal or tiny one.

As far as WiFi is concerned, I cannot access my AP when it is set to 12 or 13, with my N800 (bought in Germany) set to Germany or UK, so mine seems to be channel 1-11 limited. But YMMV - maybe that gets set up once at initial installation (a behaviour quite common on PC WiFi gear and routers), where I may have left it at the US default at that time.