1 and 5: Either should be fine, especially if the seller is reputable and has reviews. Many sells will have a store on multiple sites, but in some cases people will resell other people’s work. See if you can track down the original creator to support them 2: I suspect everyone will want STLs they won’t have to do any processing on. What do I mean? Well, I could give you a STL for a piece that’s massively too large and would need to be split into pieces or a different STL that will be impossible to print well. I suspect print services won’t want to deal with this, will charge for it, and/or you might not be happy with the final outcome 3: If you’re paying someone to print the parts, they will likely have larger format printers. However, this might cost some $$ 5: It depends how thick the designs are and how strong you want them to be (more perimeters = stronger), but keep in mind that you’re also paying for machine time and potentially processing (eg surface finishing, support removal, etc). To get a feel for a quote without buying this design, find some cosplay armor on something like printables and use that for quotes