Considerations:
1. Bokeh - the lens should render OOF (out of focus) areas in a pleasant fashion.
2. Working distance and perspective. With crop factor, 35mm is roughly equal to 50mm FF, three quarter to half body; 50mm roughly equal to 75mm-80mm FF close enough to traditional 85mm FF, slightly tighter shot, half body to head; 85mm roughly 125mm FF, head and shoulder. Any longer you'll have to shout your posing instructions.
3. Max aperture for that shallow DOF (depth of field). That's where zoom lose out.
If you're really keen on portraiture as a specialty interest, start with 50mm, very cheap, and f1.8 easily, f1.4 also not expensive. Add 85mmf1.8 if you like. These 2 would be your primary portraiture lenses. Get a 35mm f2 if fund is not an issue.
I've used 50mm f1.4 on FF (film) effectively, along with 85mm f1.8 and 105f2.5 (AIS, manual). I like the 180mmf2.8 as well, a bit tougher for use, but imho cannot be the primary portrait lens (even during the FF days, now even tougher as it is equivalent of 250mm). I love the perspective of 300mm but found it so difficult to use. I think those pro photographer uses walkie talkie and have someone directing the posing, he shots from a far (and very far). I have a 400mm f5.6 but had never shot a single portrait shot on it. Did a couple of 500mm f8 (borrowed lens) shots, nice but gimmicky (OOF donuts).
Having said that, you get that special-look on your shots if you can use lenses that are 180mm and beyond.
But my favourite (then during FF days) was 105mm f2.5, now on crop sensor AF85mm f1.8, but the AF50mm f1.8 (sold it some years ago) and AF50mm f1.4 were never far off. I also pack my AF35mm f2 as it is an extremely sharp lens.
Oh - and the Nikon soft 1 if you want the soft focus look.
My 2c.