I wouldn't build a track bike from scratch if you were a noobie because once you actually get into it, you'll look back and see what you could've done better.
I would definitely build a bike from scratch as a noob. Sure you'll look back and see what you could've done better but it's the whole learning experience. In fact, I would argue that of I hadn't had built my first fixed gear from scratch, I would probably have never kept cycling. So props to you Gabri. But I would recommend buying a whole wheel, but since you already have spokes, rim, etc. it's going to be hard to find a hub that is compatible with your spokes. Most likely, you'll have to get new ones.