No. The windows German keyboard is sufficient.

That depends on what you mean by "os-integrated method". On Windows, I'd switch to a German keyboard layout while writing German and just use that (though you have to remember where the keys are if you're not used to it). Don't type in Alt-codes, that's indeed inconvenient. On Linux, I can completely reconfigure my keyboard and make it do exactly what I want, anyway :-) I guess on the Mac one should have a similar flexibility.