Like others have said, a rear rack/panniers will help tremendously if your commutes a few miles+. Day or night as far as I'm concerned, lighting is a must. I have two rear lights, a Planet Bike superflash on blinky and a Cat-eye LD610 on solid. Front light's are great, it does somewhat depend on how often you ride at night. Either way, Don't skimp. Do get good lights regardless, But not all daytime situations call for super powerful lights. I commute everyday in the dark and I have a set of Nite Rider X2 Duals, 300 lumens and they do the job for me. I think these would be more than adequate. There are cheaper options(cygolite, among others) I also have a cheap Energizer headlamp strapped to my helmet. I cannot speak for riding in the rain. I always carry two tubes and a patch kit and basic tools(allens/mini phillips/tire levers, etc..) If I get a flat I just swap the tube and go on my way/patch the flat tube later. A pump is mandatory. Some swear by CO2, But I haven't tried that yet so I can't give an opinion. Also wear hi viz(construction vest etc..) clothing. Do everything you can to make sure the cars see you.