Ok this is one of those weird areas of expertise you only get when you live in a wet/rainy climate, commute on your board, and have like a ridiculous collection of gear.
This is my personal rain/snow setup.. Well my main one..
8.375 x 31.5 madness r7
Thunder 149 titanium with bones medbushings
Creature /OJ 56mm 97a hardline (wide but somewhat round)
Zealous ceramics
So basically what you want is a deck that you're comfortable with but don't care about. R7 has held up really well for me, i dont really worry about it sogging no matter how wet it gets. Trucks can be whatever you want but i recommend keeping them as tight as you can stand just because theres nothing worse than wheelbiting on a turn and getting tossed onto the wet pavement. Wheels I'd go with whatever size/shape you're comfortable with but wider and softer is better in my opinion. 95-97a is kind of a sweet spot for the roads around here.
Obviously bearings are the biggest issue with a rain board. I've got two kind of philosophies - either something like the zealous that are greased and tightly shielded or if you're the kind of person that thinks bronson raws are cool you can just get a set of $20 ceramics off amazon and pop the shields. Both methods work, but the zealous are actually pretty fast too.