I would also recommend running the set up before you get started. Get your timer set, and let your light go on and off for a couple days. While your doing this you need to monitor a couple things to make sure once your seeds sprout they have a good comfortable home(and they will grow)!
I know you said low budget, but one thing I think is VERY important to have(and less then $30) is a digital thermometer and hydrometer with a min/max feature. Might even be able to get one on ebay for less. Actually, for now you can get away with going to PetSmart or PetCo or some other big box store and buying the regular ones they sell for lizard aquariums. I have a set of those, they come as a pair, the thermometer and hydrometer for like $12.00
Get your set up running and monitor your temps when the light is on, and when the light is off. Same goes for the humidity.
Your going to need that 6" fan for exhaust. Set it up high and run ducting out of your closet. Your also going to need a fan speed controller. Again, your going to need a fan speed controller. The first hydro store I went to(where I bought most of my equipment) sucked and gave me terrible advice, told me I didn't need one. Well that 6" fan at full speed is way too powerful! If you can't adjust the speed of the air being sucked out of your grow tent/room then you won't really have any control over your environment!
That being said, your gonna need fresh air coming into your closet as well. You can do this two ways: with a passive intake or an active intake. I'll explain...
Passive Intake: This requires you to create some type of opening or a simple duct low in your tent so your exhaust can pull fresh air into your space. If there isn't an intake of some kind it would be like trying to suck air through a two litre bottle. A passive intake would be like putting a hole in the bottom of the two litre bottle so you can suck air straight through it.
Active Intake: This required you to have another fan. It doesn't have to be as big as your exhaust. If your exhaust is 6" you can go with a cheap 4" for your intake. Again, put this fan on a speed controller. The fan can be placed outside or inside your closet. Run 4" duct either into your space from the fan(if the fan is outside) or out of your space(if the fan is in your grow space).
Keep the exhuast high(heat rises) and your intake low. This ensures you have fresh cool air coming in from the bottom and being pulled up through your tent.
Once you have your light on and your exhaust fan running, monitor your temps and humidity.
For vegging it should be 70-80 degrees and you want at least 50% humidity.
For flower is should be 70-80degrees and less then 50% humidity.
Unless it's really humid where you live, you'll probably realize once your light and exhuast/ventilation are running and constantly exchanging the air in the room your RH(realitve humidity) stays really low. If this is the case go to CVS, Target, WallMart, etc. and buy a cheap humidifier(approx $15-$45). Put that in your space and again check your temps and humidity.
Definitely make sure your set up is running and your environment is on point before you put any plants in there.
