this one thing isn't clear to me: Is it possible, to debug C# of Unity 4.3 with Visual Studio 2013 without any plugin?
If no, what or how else should I debug then?
You can only use Visual Studio to debug unity projects if you have unity VS.
From someone who has it, unless you are part of a team and all running unity pro you could probably do fine with unity and it's built in install of monodevelop.
Mono develop still lets you debug your projects just not using visual studio.
The unity VS plugin attaches you to your game code which you can't normally do because it runs within the unity process itself (by design i believe).