For power, get a 110V to 240V transformer.
You need US TOS roms. You can get them from Simbo over at Atari-Forum.com.
You just need to change the Keycaps on your keyboard. You can get them from KLund1 (I think he is here as well but I got it from Atari-forum.com).
There is an extra key on the German keyboard, between the Z and Shift keys (US Z location). But the US Left Shift keycaps is longer and will cover the extra key. (That's what happened to mine).
Be sure that you get the US ROM before change the keycaps as the ASCII code will generate German "symbols" on the screen.
The monitor is German... you mean the volume / on-off switch are in German, right? That will not matter because you figure which one is which.