Your plants will deplete the co2 in the air, converting it to oxygen. You need to replace that somehow. The simplest way is to expel warm air from the top of the grow area and allow cool air to be drawn in at the bottom.
HPS bulbs get HOT! My 600w gets so hot I can't get my hand closer than about 4" from the bare bulb before it hurts, when it's running I can keep my hand on the cooltube all day, but that heat is being taken away from the room. If you have a fan blowing air around but not actually removing it, you are creating a closed loop and the whole basement will gradually heat up.
Ballasts don't get as hot, but hot enough that most people will opt to have the ballast outside the growroom, since it makes temperatures easier to manage.
There should be a vent in your basement somewhere, just hook some ducting up to a fan so it sucks air from the grow area and blows it out, through a carbon filter if the vent leads anywhere that people might walk past. More air will find it's way in as fast as it's being extracted, so there's no need for an intake fan.
I would think you'll be ok on temps tbh, but your babies will want co2 or they'll not grow well.