I started commuting this spring, and I went through many of the same planning stages you did.
I don't ride every day: my goal is to do 3 days per week, but this depends on my travel schedule, appointments, family obligations, etc. I plan to drive once per week to drop off fresh clothes and bring home the laundry.
I've arranged "remote access" to my work laptop from my home computer, and I don't normally bring it home except on weekends. When I bring the laptop, I use a backpack, but the other days, I use panniers on my rear rack.
I don't do any "working out" on days when I'm riding (I figure I get enough workout in 17 miles of commuting). At first I just "washed up" in the handicapped stall of our bathroom. While this works, a shower is MUCH nicer, and I've arranged with the property mgmt of our office park for access to a fitness center in a nearby building. This took some persistence, but eventually they agreed to let me use it "for showering purposes only" (as if I was going to use their treadmill after biking to work). Everybody wants to "go green", and you can use this to your advantage with the property managers. As a bio-fuel powered vehicle, you are cutting both air pollution and carbon emissions
One thing that nobody has mentioned so far: here in the Boston area, it is frequently much colder in the morning than it will be in the afternoon. As long as it's 60 or greater, this makes little difference in clothing, but below that you may need a jacket, long pants, etc. I've found that this fits nicely in one small pannier. I also carry a rain jacket. In the summer, rain pants are not worth it, but I'll use them when it gets colder.
I keep some tools (metric hex wrenches, etc) and a floor pump in my office. I also keep a spare tube (in addition to the one that rides in the seat bag on the bike) so that I never have to start a ride without a spare tube (I guess I'm a bit paranoid: I've only had one flat, and that happened at the office).
It's gotten to the point where when I do drive, I find sitting in traffic maddening. I'm much happier on the bike.
Good luck, and enjoy the ride.