Haven't painted in a while, but I've got an Artograph 1520 booth set up in a closet. I installed a shelf at a height where it's more comfortable to use while standing. I'd rather put it out in the room and closer to my work area, but I'm an apartment dweller and don't have the space for it.
The Artograph has filters so I consider it acceptable to paint without outside ventilation in limited quantities and preferably with acrylics, though I have used enamels.
I strongly recommend using ventilation where available, but if it's not available, I would strongly recommend the purchase of a system with adequate filters (my system has charcoal filters, though I sometimes question their adequacy)
Couple other hints:
* I use a cookie sheet move paints, airbrush, and other supplies between the desk and the painting area all at once rather than having to move back and forth. When done painting I move the cookie sheet back to my desk and use it as a cleaning area. That way if anyting spills, it stays contained.
* I installed a keyboard drawer underneath the shelf my booth sits on for the cookie tray as a sort of storage area while painting.