Probably the best costume idea ever. When I was little my older brothers friend went as She-ra I was so jealous. Hers was sore bought, so a plastic dress and mask womp womp.
Some sort of polyester would probably be your best bet for the cape. Maybe the kind that most store bought costumes are made of(they have a bit of a sheen on both sides).
For the muscle suit you could get a nude leotard or long-sleeved shirt (depending if you wanted the legs to be ripped too...and if you can manage to convince your bf to wear a leotard) and do some strategic shading with a slightly darker shade of fabric paint. You cold also take a note from me circa grade 7 and stuff like crazy, although I wasn't exactly going for the same look I'm sure the results are quite similar to stuffing a bra.