I've got a waterproof side pannier bag, the Ortlieb one, in which I squeeze my raincoat and my briefcase. No need to take the laptop unless I travel but that'll be mostly off-bike anyway
In terms of bike setup: 28" 27-speed trekking bike with semi-slicks, fenders, battery light holders front and back (only used during winter actually, thinking about a hub dynamo as upgrade this fall). I don't take tools or spare tubes. If I had to take a basic set, I'd take a tube, tire levers, and a Co2 capsule to inflate. I commute from one end of town to the other which takes me about 25 minutes.

I do occasionally commute on a singlespeeder (city's pretty flat) with my briefcase's flap over the top tube converting it to a frame bag. Works pretty good, you don't have a sweaty back and it's out of the way while riding.

Good luck with your project mate and keep us posted!