It took awhile, but finally I've got ADB mouse and keyboard as well as a monitor with VGA to DB15 adapter.
I've set up everything and it looks like the computer is working!
There is a German version of OS installed and nothing interesting apart of that.
I'm making plans to improve and expand... OS, RAM... If you've got any idea, feel free to comment.
I've changed my mind and because the case is in really good condition, I'm not going to do Retr0bright. I like this Mac the way it is.