I am about to plan a new char. that should be a goul from the beginning or is changed during play. He actually has Esssence 6 and Magic 5, if he is changed he "loses" 1 point of essence, but is this point really lost or could you use the lost point up with cyber without losing additional magic?

Nope, that's a complete loss. No discount on anything. It's ALL additive.

And if i have magic 5 and then put in some cyber? Do i also lose one point of magic or do i only lose it if i have essence below magic?

You lose it as soon as you install the first implant.

No, you lose magic too. Every fraction of a point of essence loss means you lose one point of magic, untill you go above 1 point of essence loss. then you lose another point of magic.

In a nutshell, no, you don't get any free Essence holes to fill with implants and you don't get to take any implants without losing Magic. Being a ghoul (for a combat type anyway) is worthwhile for magicians and adepts for the ridiculous attribute maximum hikes they get anyway. They don't need a free point of Magic or a free Essence hole on top of that.

Especially since you are pretty likely to kill a target you hurt no matter what due to you being a walking biotoxical hazard . .

Unless you're born as a ghoul, then you're actually "disease free". You still look like a freak and have crazy stats though. (According to RC)

And still receiving the same treatment of:"shoot it! shoot it before it gets withint 20m distance! Yes, even if it is supposed to be on our side, we'll chalk it up as an accident or something!"
Because there is no way to tell wether or not a ghoul is contagious to someone or not. Only by trial and error. And in this case, the trial and the error are one and the same too.