Hi! I have worked both construction and residential heating and air (yeeeeeeeaaars ago) and have grown a few plants, so that is what I’m drawing from.
If you can breathe in the room and the temp is right, that is a good start. Well insulated is awesome as well. Once you introduce lights, you will get heat. If the window unit can handle it then you should be good. You’ll still need fans and stuff.
In the winter when you have to heat it that may be different. The lights may provide enough heat by themselves depending on the size of your grow that you don’t need the heater. But the AC was forcing outside air into the room, If you’re sealed up and weather-stripped for winter, you’ll probably need air from outside circulating - to prevent mold at a minimum.
Something as simple as a HEPA filter set-up (like for a furnace) with a damper, rigged into the wall might work for that. The filter would prevent outside baddies coming in, and might reduce the sheer volume of cold air coming in. With a portable heater in front of it, it might work.
I also think that since you are trying to make it right the first time I would rig an exhaust fan in the ceiling just to be safe. In a 10x6 (close guess?) shed, I would think an 8" fan, but what you buy should have cfm information. If you run an exhaust fan, the air will find it’s way in from somewhere, probably through the AC if you leave it in the window.