You could build a box that is sturdier, way cheaper (for those of you who have read my other posts, this is how I usually begin them huh?) Probably $40 or less.
Use the HPS but would go for a 400watt light (the yield will benefit greatly from it. 600watts would be Ideal, but 400watts you can get a switchable ballast( can use MH or HPS by flipping a switch) HPS bulb, reflector, combo kit for $200 shipped from
Discount-hydro.com. Probably cost the same as the 250watt you were looking at where ever else.
The switchable ballast for single cab grow is a great investment. You can use MH for the veg time, and use HPS for the flowering cycle.
Good choice on the filter.
You will need a big fan, and it will be worth it. Check out
plantlightinghydroponics.com they have valuline (vortex knockoffs) inline duct fans, and they are cheaper than the blower fan you were looking at and put out more cfm (a 6inch fan with 435cfm, for 69.99) they handle the static pressure of a carbon filter and ducting alot better than the homedepot special you were lookin at.
It can be done on a budget, But don't sacrifice your grow, you will end up spending more in the long run if you get equipment that does not perform to your expectations.
As for a fire, many people do this and very few fuck up enough to do that. Just use common sense (the bulb glass temp can reach 700+degrees). A common thing to solve bulb heat is the glass covered, air cooled reflector.
And I am sure you mean ScrOG. SOG would need a mom chamber with room to veg clones and a seperate flowering chamber.
Hope this helps