I would go to Lows if you have one in your area. Look for the U2 wall mounting cabinet, about $30, $40. It doesn't come with a back, because it is a wall mount cabinet, they will cut a peace of plywood for you for under $15 I would think. Get 5 or 6 light sockets for $3 a peace. Then pick up CFL lights for the sockets, I believe that they carry up to 42w. Get some 3000K, or 2500k for Flower and 6500k, or 5000k what ever they have close for veg. Get a bathroom fan for $50, to $ 80, the more CFM the better, see how to make a home made scrubber on the FAQ. Get a bag of soil, one that is easy to find and fairly cheep like Black Gold. Mix with perlight, 30 percent perlight and 70 Black Gold, you can use it for first 3 weeks or so by itself without added ferts. Line the inside of the cabinet with the emergency blankets from Wally Mart for a few bucks or the dull side of foil. make some passive intake holes in the bottom of the cabinet under the bottom spacer/rim of the cabinet, put some PVC piping threw the holes which are there to allow fresh air in, place 90 degree elbows on the ends of the pipe under the cabinet to help act as a light trap. Look for black piping if at all possible, because it inhibits the light reflection. Also, put some black pantyhose on the ends of the PVC, this will help stop more light and prevent dust and bugs from being pulled threw the passive intake.
These are just some suggestions, something to think about. You could probably get 3 skinny 2 foot plants in it, or 2 bushier plants, or 2 plants in scrog with a nice yield depending on exp, or a sog of about 6 plants.
Hope this helps some.
PS, this is not a completely detailed plan, so research my plans because there are other things to consider like light proofing the cabinet, fitting the weatherstripping, heat, size of pots that can fitt inside and the small things that we all over look at the first glance of a plan.
Adieu