heres my two cents dude.
Ok, you wanna buy a growbox, rather than DIY, and thats totally cool.
BUT,
if your gonna get a box, just get the box, and nothing prefabbed with it. Buy a growtent barebones, and outfit it on your own. BUy your own lights, ducting, fans, and everything else you need to get your grow on. You would be spending way less than what you woudl spend on your prefabbed outfitted box, seriously. You can snatch a growtent that works like a charm off of ebay for 140 bucks - n thats one of the 4x4x7 tents, way bigger than your sun shed. Ive seen them in person, and while they really are nice, a tent would be more practical for growing anything more than 3 plants.
now, ill break the list of shit you can get down for under 1500 bucks.
TENT - $125-200 depending on the size u want
LIGHTING - 400w $150-$225 , 600w $250-$350
Ducting - $19 for 25 feet of ducting
Fans - 1 Vortex fan - $130 bucks--one 80cfm blower $19.95
n the rest is growbags, nutes, n soil. i outfitted my whole setup for somethin like $650, lighting n all - but i found some awesome deals on Ebay. You could too though man. Just try n save as much as you can, there are other growrooms out there that are aLOT better than this one - i know its cool cuz its in a steel casing and has a lock on the door, but growtents can be locked aswell.
Just think before you jump brother.
N good luck!!
-K1.