Guess it all depends on where and when you are biking.
I used my bike to and from work for half a year(not during the winter) distance around 17 km one way. Quite heavy terrain, had to carry the bike over some big logs and rocks on the forest path. Since this was in sweden I never had to use it when it was dark. My employer supplies me with work clothes and there is a shower at work.
My equipment(which worked out great and i think i used all of it):
Bike: Swedish military surplus bike from the 60s.
(No gears, lockable toolbox and superior quality)
½ litre of water
Poncho (to keep dry when it rains)
Patch kit, spare valves
Cell phone (when i eventually got a puncture i could call work and tell them i was going to be late)
Towel

Only thing i thought about that I didnt include(because of its weight) was a spare chain..

When it comes to the bike itself I think quality is priority number one, if you want something fancy and quality it's expensive. (Never found a mountainbike with the same quality as my old one for less than 10 000 SEK
(approx 1500$).)

Good luck!

/Petter