I would look up some thermoplastic demonstrations, like Worbla for example. Theres also a thinner transparent version out there too. Great for armor and props.
Fiberglass is another method. People take sheets of foam (home depot) shape out their armor pieces and fiberglass (also home depot) over them. I’ve seen it done with paper too, look up pepakura and fiberglass.
For the mask, latex. Heres a demo, it might look like an intimidating project but it’s also alot of fun. Just need some calipers to make sure everything is symmetrical.
Then you can either go fabric shopping or buy a black/grey leotard from an athletic site or store.
Also, for a personal sized mannequin find a close fitting t-shirt you dont want. Put it on. Wrap duct tape tightly around your everywhere there fabric. Remove by cutting a seam up the side of your shirt making a duct tape shell of your torso. Tape up the side and stuff it.
As for the muscles, tough call. Most muscle suits look terrible, like a children’s hulk costume. I think there male “spanks” that give the illusion of shoulders, pecks, and glutes, not sure though. I reccomend a diet of steamed chicken, broccoli, rice, and water with 2 hours or rolling a medicine ball up your body, over head, then back down slowly, alternating toes every 5 rolls.
Try to use light fabrics that breath so you dont get too hot. Hope this helped