im new to the whole fiberglass scene myself, but heres what ive gathered through my studies:
1) i know the resin likes to stick to fleece, so maybe that? thats what people use if they are just building a box from fiberglass at least.
2) cant really help you there.. sounds like googling territory to me.
3) ive read that when making a fiberglass enclosure, you put enough layers on it to where you can stand on it without it flexing. dont know how to measure that if youve already build a box from wood tho. -shouldnt need too many layers since the MDF is pretty strong stuff.
>>on a side note, why dont you just get some sandpaper/electric sander and round off the edges and paint it? that would be MUCH MUCH easier and would probably look better in the end. if you wanna fill in any gaps (i.e. inbetween boards and what not) , i recommend epoxy putty by Loctite (found at lowes). use that to fill in gaps and then you can sand it down. sorry im not an expert, just figured my answer is beetter than no answer