Yes, using your own well-fitting clothing as a pattern is maybe the surest way to get it right. Hopefully, you could do it without destroying the current clothes. Seam allowance could be anywhere from 1/4" to an 1" depending on what type of seam you'll use. A little destructive testing on beater garments can be instructive.
A quick check (for instance in a thrift store) if a fabric is waterproof or not is to *try to suck air* through it. You won't get a psi rating that way, but you can breathe through an uncoated fabric but not a waterproofed or membrane fabric.
Fit, slim people are a discriminated minority in this country, at least for mass-market clothing. Even at 170 pounds, 6-feet tall, I'm loose in some "small" garments. Small tall is often the best choice, but usually tall options don't even include medium, only large and bigger. Duluth Trading has their "longtail t-shirts" (to prevent "plumber's butt") in small and trim and all of them are long. In recent years, they've added wicking fabrics, long sleeves, pockets, v-necks, etc, so it is a potential alternative to expensive brand name outdoor wear. And an option for those only need to lose 15 poinds, not 100.
Editted from "breathe through it" to "suck air through it" because I was unclear before. David Olsen (below) is right, that's how to check it.