Apple Watch Ultra is cellular-only. There is no non-cellular model.
And your Ultra 1 would then be used or new-old-stock, and might not have full battery capacity.
The Apple warranty and AppleCare coverage (on a new or Apple refurb) is not international.
A subset of features are available without a paired iPhone:
(I’m not sure if mail is available. The mail support was once dependent on the iPhone. Maybe that got fixed?)
An old Apple Watch Ultra 1 can have pulse oximetry support, but newer US watches will only have that via the paired iPhone.
And yeah, whether it’ll be acceptable depends on the German telecom involved.