1) Breakfast - my commute is 12 miles each way, and I tend to ride strong and steady. I have a fruit and whey protein smoothie when I wake up, and have started packing a couple of boiled eggs to get a mid-morning protein bump. I try to eat a little something before the ride home.

2) Tires - Pass - I have nothing to add here - others have covered it

3) bag - Backpacks suck for anything more than a few miles. Seatpost racks CAN be a pain with any more than a few pounds in the bag - it is possible for them to swing or slip off center. I generally don't like them. If I were you, I'd keep my eyes peeled for a good steel-framed rigid fork mountain bike or a cross bike on craigslist (or other sources) and build up a commuting specific ride. My sister lives in Minnetonka and I've visited in Winter - I'm pretty sure the Klein would be hard to convert to a winter bike (probably not enough clearance for large studded tires, fenders, etc).

So, my advice would be to make do with a backpack and save your money for a used bike and all the stuff you'll need as winter comes back around.

Congrats on the job, and welcome to the wonderful world of bicycle commuting!