Relax man. Growing marijuana properly while making it stealth takes some basic knowledge. Utilizing experiences and discoveries of the community can only benefit you. I strongly advise you to not have the attitude of throwing money at the problem. If you want to pull off a good stealth grow, all the knowledge on here is priceless. This forum is a mass of individuals that think its bullshit that we cannot enjoy a nice smoke at home due to some redneck senator's interest in a timber farm whose influence still sticks today.. unfortunately. As such, they have figured out LOADS of ways to hide in plain sight. Read around some more and learn. You need to know the components of HID lighting, concepts such as light-proofing, learning to hide cords, etc.. There is a reason for such painstaking DIY taking place. Not to see that prefab is bad, just saying I think that you need to learn some more before jumping into this.
With a single google, I found HTG has a all ready to go CMH 400W kit.
http://www.htgsupply.com/Product-HTGSupply-400-Watt-Ceramic-Metal-Halide-Grow-Light.asp
If you are after a 250W setup, just buy the bulb from advanced. Sorry to tell you this, putting together a light involves screwing in the light bulb into the socket. You plug the fixture into the ballast.. and walla. Its pretty simple and you will get a great light for your purposes.
As far as your enclosure, you're going to have to do some work to hide all the wires anyways. Also are you doing hydro? Soil? Soil-less media with nutrients? These will dictate how much is going to have to fit inside and how many wires you are going to have to hide. I think the best setup I saw on here for stealth was a U-haul wardrobe box. The light was a 250W I believe and didnt heat up much. Some 120mm PC fans were used to exhaust. It looked like just a box sitting in the corner.
With just a 2x2x4, You could EASILY make that out of plywood and even a hand saw. I have no carpentry skills what-so-ever lol but that seems do-able. WIth that size also, I would look around at various plastic Rubbermaid cabinets or similar and find one for the size you need. Good luck!