1- No matter where you place the fan, it’s no closer than the one you’re replacing on the back of the case. You won’t be too close, just remember to turn off the internal fan.
2- Your Glowforge is an indoor pet. It can’t tolerate extreme temperatures (<40F or >120F) or extreme humidity (<10% humidity or >75%), per the operating environment chapter of your user manual. Whatever kind of air conditioning keeps the humidity in that range is what you’ll want.
If you’re going to leave the machine hooked up to the ventilation permanently, you’ll want a blast gate to separate the indoor from outdoor air when the fan isn’t running. The louvred vent cap alone won’t do that job. You can print one using your Glowforge.