1. Take test rides- a bunch of them. Gears or not is up to you.
2. Slicks for road, knobbies for dirt/snow/mud, spikes for ice.
3. Buying used is not a bad idea, provided that a) it fits and b) you have someone knowledgeable give it the once over before buying.
4. Expect to get wet no matter what your rain gear. Layer up with wool, avoid cotton, and use a nylon shell to block wind/water.