The Sigma 18-35mm f1.8 is a unique lens, if that is what you want, get it!
But if you want lower weight / native lenses, then it means compromises.
Here is one path I might take, a simple two lens kit:
1) a mid range zoom which is f4 (or f2.8), I use this when I need to R&G and don't need that super lowlight
2) Panasonic 25mm f1.7, for those times when I need that extra speed! (but lose the flexibility of a zoom)
Between those two lenses you could do a lot (for instance the zoom for daytime, the prime for night shoots!).