I was going to get a saga kit for my first guitar too, but I ended up buying all the parts from a local music store. It is much more expensive, but this way I'm sure I'm getting all high quality parts(I hope I don't screw it up!). But if you want to keep the costs low, tha saga kit seems to be the way to go (I would probably have gotten one of those if I lived in the USA, but shipping to Spain costed too much: $75 for a $100 kit)
I suppose that the headstock is sized so that the most common Fender shapes fit.