how do you feed your plants nutes? do you mix ferts with the exisiting soil or do you spray onto the leaves with water?
You just mix the nutes up with the water you give the plants. Don't give nutes until the plant is 2-3 weeks old and has 3-4 sets of real leaves (not the cotyledons) and then only use a low dosage, 1/8th to 1/4 of the strength on the bottle depending on the brand, and gradually build it up so you don't overfertilise.
Feeding schedules vary from grower to grower, I like to give my mums a good dose of nutes every watering, then flush once a month to try to prevent salts building up in the soil. I feed my clones every other watering, building up the strength until they go into flowering when I switch to bloom nutes and feed every other watering, increasing the strength every couple of weeks until harvest, and flush after 4 weeks of flowering. I don't bother with a final pre harvest flush, and there's no difference in taste, or potency, as long as it's cured right.
2x3 isn't that small a space, dependant on the height available, but if you've got 4' of headroom you can grow a couple of good little plants in there. I'd get a switchable HPS/MH, 400w will be more than enough. You could use a 250 if you're worried about keeping the temps down, and it won't do badly, but you'll get to the end of your first grow and wonder what difference a bigger light would have made imo.
You will need to extract stale air away from the top of the closet, into the attic if at all possible but if not then anywhere you can inconspicuously duct to away from the room. Also required is a fresh air intake, this can be as simple as a hole with a flap over it so air can get in but light cannot escape/enter.
To maintain correct soil ph you just need to test your nute solution before giving it to the plants, then test the run off and adjust future waterings to lower/raise soil ph as needed.
Any more questions?