I would opt to buy a tent if that's an option. 3 feet wide is plenty, but I would go up much higher. You need to figure out a budget. Look at how big those fanfilter combos are. Unless you can figure out one of these DIY filters and actually make it work (PC fans and booster fans won't cut it), once you spend the money and size on a pro-filter/fan, you might as well go big 600w HPS, cool tube, ect. If this is your first grow, get a managable size set up, throw some CFLs and PC fans in it and grow 2-3 plants knowing you are going to chop at least 1 male unless you order seeds. See how bad the smell is in your place during flower, try covering it up from there, make a bunch of mistakes, make adjustments and see if this something you want to invest more into. You are going to want fans inside the box blowing across the plants as well for ventillation and thickening up the stems. Again, space is the worst enemy. I have an engineering nightmare myself trying to do too much with too little room. The more room, the cleaner and easier.