New Setup... Please give opinions :)

Hello... This is my first attempt at making my own box setup... any help would be greatly appreciated cause Iâ??m looking forward to doing this right the first time :)
The picture included is a basic layout of what Iâ??m looking to build that way you know the dimensions Iâ??m working with

First I was wondering how many plants I could fully grow in a 2 1/2 by 2 1/2ft room that is 5 ft high... Next what light should I get (how many watts??) I was looking to use only MH for growing and then HPS for flowering and is 5 feet high enough from the plant to the light to use those lights???
This was the one unit I was looking at
LINK TO LIGHT
I wasn't sure if digital ballast is really worth it and I am not sure what watt system I should get :(

Next problem I think I might have is the air purifier that I have.... would a mini air filter unit you can get at radio shack suck enough air to cool the light at all and filter the air so it doesn't smell... a 6 inch ventilation hose is going to be taped to it air tight.

Last but not least... does anyone recommend what I should do to make the temps in that room lower...? I have a feeling its still going to get to hot and the box will be completely enclosed.. The only way I can get to stuff in it is to unscrew the front wall (which is what Iâ??ll have to do everyday)
I'm a little worried with an enclosed area with that lamp might not be safe for the light and it might get to hot for the plant (any feedback on this would be awesome) I think I need something else so the plants get some fresh air tooâ?¦. Thank you so much everyone for your help I canâ??t wait till I can actually show this thing being built and put it to use!!!

nice, how many plants? thats a nice light, if you can afford it. you can fit up a system that sucks hot air out and bring cool air in from outside. Co2 allows you to have higher temps

i was thinking about 4 plants in the end but i'm not a 100% sure... thanks for the reply lol i was kinda surprised when i saw someone posted on here