First off... welcome to the forums from a fellow european.
Since the Mustang is a lot less common here then it is in the US, the maintenance and modifications are in tandem with that. Basically that means: You're gonna end up with someone that probably has never seen the car, or done that specific modification to it.
As to parts, those I'm pretty sure will have to come from the US. So factor in shipping aswell. Since the Mustang isn't a european car, you can rest assured that while they do have some parts on this side in the Ford warehouse, the more exotic the part is, the less likely they'll be to have it. And I'm pretty sure they do not have a turbo lying on the shelf
As to installing it... like I said, you'll probably end up with a shop that doesn't know the car, or has never performed that specific modification. So that means it's pretty much back to trial-and-error. Can't give you advice on the availability of said shops in Germany (since I'm not a resident there), but there's bound to be someone that can on the forums. Don't get your hopes up too much tho. The Mustang is a US car, and not an EU car, so anyone doing that kind of stuff is far and few between.
Leaves the question, if you're US side now... why not install the turbo's there before you move to Germany? That way you'll save yourself much of the shipping and know it'll be done by someone that atleast knows most of the engine and how it should be done. Just be sure not to tune it down to specific gas octane levels if you do decide to do that. Most common fuel here is 95-octane.
Hello / Hallo / Hola 
Thanks 
