here are the "must haves" that I recommend-
Core Lightweight hoody- a really nice lightweight base layer, utilizes the latest grid technology so it really wicks moisture away- the hood is really handy
Core Heavyweight hoody- a very nice mid-layer in colder conditions, could serve as a insulating layer in warmer conditions- also uses grid fleece, just a heavier fabric- it's cut nice, the hood fits great and just works
Kelvin Light hoody- synthetic insulation so a little more insurance in moister conditions vs down, very good weight to warmth ratio
with those three pieces you would be set, save a shell layer, for your upper body