there are some basic things you should consider...
1) your grow size is not defined by the number of plants but by the area and height of available space for growing. naturally, your lightbulb also depends on this. so first thing you should do is decide what area is available for your growing purposes and how much vertical space in that area that you have. it all depends on that. you can easily fit 25 plants of the right strain comfortably in one square metre (if its your first grow i'd stick to 15 though) in a SOG style, or you can grow one plant in one square metre SCROG style, and then again there's the option of growing 40-50 per square metre in a VSOG, which i would not recommend given our humid climate.
2) likewise, your ventilation source and the size of your fan that you use is dependent on two things: the size of your room, and how you plan on ducting the fan (if you are ducting it, which i frankly think is a waste). remember, ducting with bends and curves has a massive impact on airflow through the duct so remember to compensate for this.
3) I would like to use a dehumidifier for my grow. Unfortunately, at the moment, i'm testing in a 100 sq foot room. I say testing as I am only utilizing about 12 square feet of space under (or rather around) 1 400w HPS bulb. The problem with a 100sq foot room is it requires a massive amount of airflow. If I have such a huge amount of stale air being pulled out and fresh air being sucked in, unless i route the intake through the dehumidifier, it makes no sense to have a dehumidifier as fresh (and naturally, humid) air from outside is always coming in while the stale (dehumidified) air is always being sucked out. I would require a ductable dehumidifier to route to the straight 6" intake duct that i've got and those are upwards of 40-50k. a portable dehumidifier just sitting in the room is not an option, as for the amount of airflow i've got, i would need a dehumidifier that costs around 30k and it would be perpetually running in an environment it's not designed to run in and the elec. bills would stack up.
However, since your grow is much smaller, i think a dehumidifier would go quite nicely if you're willing to make the investment. remember, you'll need one that is intended for about 5 to 6 times the volume of your grow zone. so a dehumidifier intended for about 100 sq feet should serve your purpose nicely.
4) to construct it yourself, I would personally advise you to build a wooden frame and cover the outside with tarapulin, and use a reflective surface (i personally like buying the things people put on their car windscreen to reflect sunlight) for the inside.