I need a full unix/linux environment with ca. 100GB of disc space 4-8 cores 4-8 GB of RAM and an Internet domain. Is there any service that you know that can rent something like that?

Many hosters will offer preconfigured servers that you can rent on a monthly basis and for a monthly fee. You're from Croatia, right? I can't recommend any Croatian ones, but df.eu has a good reputation if you don't mind doing business with a foreign company.