What is the meaning of a mobile operator version 000-IT (I think it's italian)? Can I use that windows phone even in Germany or have I to change the operator? And when, how can I change it?
That is a carrier customization identifier (I think it's called a CSC when describing ROMs; I don't mess with those). The first part identifies the actual carrier (
000 means it's not specific to any carrier). The second identifies the country that the phone was made for (
IT is, as you guessed, Italy).
I see no reason this shouldn't work for you. The fact that it's not carrier-specific means it won't be SIM locked or anything like that. Being designed for Italy it may default to Italian language, but you can change that easily enough (it should ask you during the initial setup, in fact). So far as I know, there are no nasty regulations in Italy that would require the phone to have something on it that you wouldn't want, or to be missing something you would want (a lot of services are unavailable in China, for example, so Chinese phones tend to lack the ability to use them).
The only really likely problem would be a mismatch of mobile frequencies. If you use different bands (say, 1700MHz instead of 1800MHz) and the phone supports one but not the other (which is unlikely; I think pretty much all of Europe uses the same bands and new phones usually support all common bands) then you could have trouble.
For the record, if you really want to, you can probably re-flash the phone with a ROM from your carrier, or at least from your country. The only common reason to do that is if you want some carrier-specific customization that the phone didn't come with, though. You would use Nokia Care Suite, or whatever it's called now, to do this for a Lumia; for another model you'd have to find the tool and the ROM to flash onto it. Sorry for the vagueness, as I mentioned, ROMs aren't my thing.