Okay, let’s be clear, the single most important piece of gear you need for riding in the rain is a good set of lights. Everything else is secondary. Beyond that, I would definitely get a good waterproof jacket and some warm, waterproof gloves, and install some fenders so you don’t turn your spinal column into a dirt road. Everything else follows by point of preference on whichever body part you’d most like to keep dry.
It’s all well and good if you arrive at your destination dry, but the enthusiasm will be short-lived when you realize you’ve put your laptop through nature’s wash cycle. So whether your preference is panniers, backpacks, or messenger bags, finding one that’s waterproof is key.
Ortlieb, Vaude, Detours, Banjo Bros, Altura, Green Guru, Novara, Chrome Industries, Thule, Rickshaw, Timbuk2, Two Wheel Gear, and Swift Industries all make waterproof panniers, and you can similarly find waterproof backpacks and messenger bags at Osprey and Mission Workshop, as well as most of the brands listed above.