I am wondering how strong they will be when they are flowering and what to expect? What precautions do I need to take?

If stealth is very important to you, you must not let people into the room where your grow is, and in that room you must use as much odor control as you can afford. I would use a big carbon scrubber with a lot of air exchange pushed or pulled through it. I do this in a closet in a room where no one is let into. In the room I have an air purifiyer during flowering. Depends on your strain, but you may need a big air purifyer.

Remember noise and light are also major stealth concerns.

You cannot grow where people will be and get away with it. You are stoned or otherwise not thinking clearly if you think otherwise. I don't care how stealthy your cabinet is. And like I told some other guy, if cousin Randy finds out, then your whole family will know. Both sides. Trust me.