I'm Austrian (native German speaker) and I can tell you, I despise typing on UK keyboards ... the layout is quite different in my opinion, leads to lots of typos or incredibly slowed down typing especially when it comes to the symbols, so you might experience similar troubles with a German keyboard. Also you'll have (for you) useless letters like Ä Ö Ü and ß hanging around, wasting space.
I even went as far as to order a base model rMBP online, waiting for weeks without a computer for it to be delivered (instead of quickly picking one up in an Apple store here in London) because when you order them online it allows you to choose between a UK English, Arabic, German and French keyboard and I was not willing to settle for the UK English keyboard they sell in store.
So yeah, don't underestimate the keyboard, but perhaps that's just me and you are more flexible in your typing.
Edit: I suppose since it is an iMac you could easily exchange the German wireless keyboard for an English one down the road. Ignore my post then. ;-)