Im trying to play a no killing/stealth playthrough. What should I do and are there any tips?

Invest in sleeping darts. Invest in upgrades that maximize your stealth, such as Blink, Possession, Dark Vision,
Agility and Bend Time. As soon as you're able, (when you find the plans) buy Boot Stealth 1 and 2 which enable
silent running and sprinting). Optional upgrade: crossbow range.

When entering new area, seek out higher ground and spend a few minutes observing the area, taking note of patrols
and which routes they take. Divide and conquer. Stash the bodies somewhere where rats can't get to them.

I agree with everything here. I will however add the crossbow range, This will help when your high atop a building or overhang so you dont have to close in to be effective. Also Mask Zoom, this paired with the crossbow range is brilliant because you can aim while zoomed This helps due to the fact that the auto aim usually aims for the head, with the zoom you can manually put it on their leg or arm. Also if you sleep dart a gaurd on the roof wait until he is in the ideal spot to dart him, ( I have made this mistake ) and darted him while he was walking torwards a ledge, I didnt judge the distance between him and the ledge and when i darted him he fell off the building and died I had to start over. And last but not least If you pair Bend Time with Blink you can pass through crowded areas without anybody knowing you were there, This is better than trying to blink from cover to cover with the chance of being spotted.

Hope this helps.

I would avoid Dark Vision as it's so much more thrilling without it, as it's quite a powerful power consider stealth play will be your focus. You want some challange.
Use Blink + Agility effective and perhaps go for either Time Stop or Possession.