Start with a really good waterproof shell.
My specs for a shell include: water-proof (not water resistant), hood (preferably adjustable), zippered pockets, slightly longer than your bottom (so water rolls off past you instead of onto you). Sometimes finding something that ends below your pants pockets is the hardest detail to find. I also like the pockets and zipper to have flaps to keep water out. Adjustable cuffs are good, as is a strap to tighten the wasteline to keep the wind out. For warmer weather, a back vent and/or zippered underarm vents are a plus (but probably not for your trip).
I currently have two shells. One is very light weight, for summer packing, not much more than a water proof wind breaker with lots of venting. The other is still considered packable (folds up into a smallish pouch) but it has some kind of lining to retain heat inside (like the "blankets" they give you after you run a marathon).
Under the shell, you will want warm layers. In April, I'd pack two that can be used either seperately or together. Right now my go to for cooler weather is a lightweight merino wool sweater and a packable down vest.
In various combinations the three can take me from a blustery dry day to heavy rain to freezing temperatures (but then I add a knit cap, leather gloves and/or a scarf that I purchase locally rather than pack).