500-600 bucks is very little to do what you want. even though you said you couldn't build; FYI, the kit you are looking at is a simple system, can be built for around 100 bucks. If you can buy a 3.5"holesaw and drill 6 holes in the top of a tub...you can purchase everything in the picture
for around 100 bucks, and you don't necessarily need the water pump, it is a little better , especially if you are experienced , but you don't need it! That's all they did...Drilled 6 holes, and threw together a little nutrient package, and some mushroom powder or whatever

If you can buy some panda plastic, and screw screws into 1"x2"s you can easily make a box, but, you must have good ventilation, if this is going to be enclosed. You have to have a way to keep the heat and humidity inside the box low enough for your plants to grow. That is where the big expense comes from (aside from growlights, they're not cheap either). Good quality Fans are rather expensive.
Goodluck