Is odour going to be a problem? If so, then you should plan ahead for that (carbon filter/scrubber, etc).
Are you going to have a way to adjust plant/light height? Do you have enough exhaust to keep that 400watt bulb cool? Do you have enough fresh air to supply the plants with not only C0[sub]2[/sub], but a light breeze to blow over the fan leaves of the plants.
Next, do you have a way to control moisture in the air (relative humidity)? You might need to invest in a small humidifier to keep the air at a nice 45-50% during vegetation.
How is access to the plants? Just something to consider when setting up the room. Make sure that you can inspect/remove them without much hassle. This will prevent damage later on as you might frequently remove them to inspect health. I hate reaching into my tub system to get at the plants.
Are your closet doors light-proof? Are you going to be able to keep the 12-hours of perfect darkness for flowering? You might consider getting some green light bulbs as you can work on the plant under green light (helps when you have work/commitments and can't get to the plants during the 12-hour day).
Just a couple things to mull over in your head when designing the room. One last thing, if there are any fans in the room, make sure they have proper guards. Hands/plants/whatever can get damaged from contact with the blades. Even PC fans can tear a plant to shred in seconds.