OM - I dont think you will need a carbon air scrubber for a 250W grow. There will be some smell, but seriously if you get some glade plug ins and cook something spicy when the family comes over you should be ok. Also - I think the open door ventilation will work using two small 8-12" fans. This is what I would do - take one of the fans and blow it directly under the light - use an old chair or something to keep it at the right height. Use the second fan to blow air out. Try to run the lights at night when it is cooler so your temps dont get to high and people arent as likely to come through. Is someone gonna be home to close the door when you are in 12/12?
The truth is this - unless you want to be there opening and closing your door every twelve hours to block out the light you are gonna HAVE to ventilate using other methods - if you dont it will get too hot and your buds will turn out shitty. I have attempted this EXACT same grow before in a slightly smaller space (a cab) but I ventilated with a squirrel cage fan and honestly - it still got too hot and I wound up with weak nugs. The resin glands do not develop right when the temps go high. The main problem you will have is not smell - it is the ventilation.