Hello and welcome! For a simple intro build with good instructions and materials I would recommend the andrewdft odst on YouTube. It may not be 100% game accurate, but will walk you through every step of the way and give you a basic understanding of the process.
Unfortunately almost every way to build a cosplay has some sort of hazard to it, be it knives, chemicals, fumes from superglue, or painting. Most of these arent too dangerous if you work outside or in a well ventilated area, but a respirator would be very useful. Sure there are ways to work around this and use other methods or materials, but they may not work as well or achieve as high of a quality as the somewhat hazardous materials. I'm not saying it is impossible to glue your armor together with the adhesion and precision using hot glue instead if contact cement, but the later would probably do a better job. There are many threads on the forum explaining different materials and how they work, as well as builds going into depth on the building process so you understand what's going on. Foam will probobly be your selection. Its durable, cheaper than other materials, and works well. However if you really want to save money, cardboard could be a good substitute for a prototype.
As for 3d printers, the larger the print bed and quality of machine, the faster the price goes up. Some good budget machines with a large print surface are a tevo tornado, cr-10, and ender pro
I hope this helped!