No one is born a witcher. I’d rather see it as a second lifepath, or quite possibly second and third. That would also let you space out the traits and get witchers actually old enough to have finished training—and not after having spend their entire lives doing something other than being a witcher.
Witcher Candidate might include the Choice and the Trial of Grasses, and probably give Keen Senses. Witcher could give Altered Metabolism and Nerves of Steel. I’d make the code an optional trait. After all, not every witcher is going to have similar codes, or even any code at all.
But actually, for a new GM I’m going to recommend something else entirely: don’t have Witcher lifepaths. I’d strongly advise against an explicitly Witcher game; either you’re going to have some people play witchers, and therefore be more important, or everyone will, and you’ve just forced a lifepath on them. If you must have witchers give them the traits for free and let them use the standard lifepaths at will, but I’d even argue against that. Suggest a game about being outcasts who fight monsters and let them decide what kinds of characters they want. Maybe one wants to try a peasant-soldier who’s turned monster hunter, another wants to be a knight of a militant order who is dedicated to fighting evil things in the night, and a third wishes to play a disgraced scholar who is really researching monsters and killing them for samples. Don’t lock your players into the game you’ve preconceived. It can work, but the game will be better if you pitch your idea and then let players pitch ideas back at you.
If you must, give everyone free witcher traits. Don’t make witcher lifepaths. A party full of the same thing is boring.