I just made my first fiberglass piece not too long ago; it's not hard, just time-consuming and a bit messy. There are no doubt loads of tutorials in the prop forums -- just do a little searching and you can probably find all you'd ever want to know about the stuff.
One way you could go about it is to make the basic form with found materials and/or cardboard. Then fiberglass it -- it's mainly soaking sheets of fiberglass cloth in resin (both can be found in the auto repair section of WalMart), and then covering your form. Think of it as very sticky paper mache.
It's one idea worth looking into -- the end product is light and durable. You just have to have the outdoors workspace to put it all together and the time to sand the finished piece.
For the jumpsuit: Loads of safety supplies
places sell coveralls -- they're close to the pyro jumpsuit, if you don't mind them not being so baggy (though I suppose you could get them a size too large?). Some even come in red or blue, then you just have to stain/dye the legs. You might find some for cheap at a local surplus place.