1. You have to decide that. I like commuting on a road bike with drop bars and gears. I use to commute fixed as well, but gears makes more sense if you have hills and it's raining. Fenders are a must in the rain, so ideally you want something with clearance for wider tires and fenders. Braze-ons for fenders and racks too, but you can surpass that with P-clamps.

2. I like 700 x 28 tires. It doesn't snow much here so Panaracer Paselas work fine for me. I like a fast commute.

3. Buying used is a great idea, but you have to know what to look for. You have to be able to spot and judge damage. Buying used can sometimes be a money trap.

4. waterproof and breathable.