Carbon filter will kill the smell...exhausting through a "filter box" (which is a homemade rig) will allow you to blow through rather than suck, but it masks the smell instead of neutralizing it. Smell isn't an issue, so long as you have the space for a carbon filter or a filter box, and to rig them properly. Heat venting is a MAJOR issue, though. For 250 watts HID, you'll want to keep it down to a small tent...a 30" by 30" max, really (2.5 by 2.5). You COULD use it in a bigger tent, but not effectively enough. Honestly, since you are getting realistic, and going for a tent, now, you'd be safe running a 4 by 4 by 6 under a 600 watt HID, running between 4 and 6 fast autoflowers (8 week veg) in 5 gallon buckets, and using the last bit of room along one side for a carbon filter, then drawing the air through the duct and out away from the tent. A bit of "fancy work" and some paint, you can even use it to make a radiant heating system for a room, though it won't be the most efficient thing in the world.