1. the quick answer is no, but contrary to what danny boy thinks, not all non-lfs
epoxies are unsafe. in most cases, the fish suppliers are simply taking a non-toxic product that is already available and marketing it towards the fish people with a hefty markup. case in point: "aquarium-safe" silicone. while someone might pay $11 for a small tube through their lfs
or an online supplier, i can get two to three times more for $4 at home depot. it's called GE Silicone I (or the DAP equivalent.) that said, not all epoxy products are created equal. some have different solid content and some will leach toxins, VOCs, and other poisonous compounds into the water. for a background, a high solid, non-toxic epoxy is the way to go, obviously. the thing you want to look for is a NSF 61 rating. this means that it is approved (usually by the FDA) for use in potable water situations. public water supplies and water towers commonly use high solid epoxy linings, as do zoos exhibits and public aquariums. i used sweetwater epoxy paint for my new DIY
aquarium and had excellent results. i have also heard very good things about the west systems boat/marine epoxy.
2. i have not built a 3D background, nor do i intend to due to my lack of artistic skill, but there is a ton of info/instructions about building them here --> http://www.cichlid-forum.com/articles/diy_list.php
a word of caution: not to discourage you, but your final product will either turn out looking very believable, or it will look like styrofoam covered with epoxy. the commercial 3d backgrounds and exhibit backgrounds are created by very talented artists. from all the DIY
backgrounds i have seen people build it, is either one or the other. rarely is there a mid-ground. the end result will depend heavily on what look you're going for and your artistic ability to actualize it. best of luck.