83 degrees will work for the seedlings, but ill let you know right now that adding more lights is only going to increase that temp ... and it will be too high. you NEED ventilation.

I am rarely pessimistic about things but i know without ventilation things are not going to go well ... the plants need air exchange as well as a temp not much higher than what you are at now.

Are you hiding the grow from someone? Or are you more worried about changing things structurally? If the closet has a door, take off the current one, buy a new one and cut a hole in it, put in the dark room vents if you have to. You can just throw up the old door later on.

Just my :twocents:

Take care and good luck ...