Sigma 17-50 f28 is great lens, not just for the price.
It is moderately wide at17mm, but it can be interesting for portraits at 50mm and f2.8, it can be useful at night as well.
This lens is very sharp, it is not super sharp at the corners wide open at f2.8, but for landscapes at f4, f5.6, f7, or f8 it is sharp all the way.
Build quality is good, I only do not like focus ring that rotates while auto focusing, but it takes one day to learn how to hold it, and zoom ring turns the opposite way, it takes 3 days to get used to it.
It is very good lens, I do not know if it is wide enough for you, and if 50mm suits your portrait needs, but for me and my photography this lens is great.