, i want to experiment with a 3D modeling program i have
Do some tutorials and make your own, bungie gave you all the tools you need in theatre, depending on how much time you spend doing it, a model made from good reference images will be just as good as an in game model, sometimes better, because you can model it with good topology for pepakura in mind, sure its not as "authentic" as an in game mesh, but once its covered in bondo its the finish and details you give it that will make or break it, besides even the in game pep files floating around this forum have been heavily remodeled(the good ones have at least), so in game authenticity is a bit silly anyway. A pep helmet based on an in game model can be total rubbish, or completely amazing, the same goes for customs, pep is just a skeleton.
If it is COMPLTETELY fan made, then yes, it is legal.
Im not sure about that, any file that bore the distinct likeness of a copyrighted work or character would still be the original designers intellectual property, and covered by the original designs copyright so, you'd still need their permission to distribute it. I doubt they'd care though, otherwise sites like scifi3D and turbosquid would get shut down.