almost every single question you have or may have has been asked and answered before.
You can use both light and heavy mats, the end thickness should be around 1/2" or thicker. The box when done and cured should be strong enough to stand on.
For making the mold, usually 2-3 layers is acceptable before removing the mold from the vehicle.
For how much to order, measure the length and width in ft., and multiply by 1.25, then multiply this by the total number of layers. This will be the minimum that you would need to get to make the box, get a little more than this, and you'll be fine. On the fiberglassite.com website they have a link that shows how to calculate how much resin you will need. It will seem like a lot, but you will need a lot. As a beginner, I'd also recommend getting the roller that they sell too, it will help get the bubbles out. (Bubbles are trouble in fiberglass boxes.)
Hope this helps more. And BTW you don't need to quote the previous thread if that's what you are replying to, quoting threads helps when the quote is from a few posts back... Just trying to conserve...