If i may propose a different idea,
while the sigma 18-35/1.8 is a stellar lens, there seems to be a high degree of sample variation with focus-errors (thus you may have to also rely on MF there), also its quite an expensive option.
in your position, i would consider a fast 50mm like the FA/1.4 (it gets you the option to stand back a little) alongside the tamron 17-50/2.8. you can find those two used for much less than the sigma and it gives you more flexibility.
(in case you wish to go wider, i can recommend the pentax da 14/2.8, it has soso critics, but i find it especially fun for people photography in narrow spaces)
hope that helps