I agree with the dry run first. Also be aware of how you feel after you do it. Make small adjustments as necessary (raising/lowering saddle, adjusting tilt, fore/aft position, etc.), but not all at once. Think about what you need to bring to work, and what you can store their as well. If you sweat a lot or don't want to feel scumy have baby wipes available.

A Backpack might be fine at first as well. If you have the money, there is nothing wrong with having a rack. Also learn how to fix a flat, before you need to fix a flat. Learn basic bike maintenance as well. I would hold off on all the accessories until you know you will keep doing it. A rear blinkie like the Superflash, and a front blink or steady light should be enough if you are not commuting at night.

Anyways good luck, and let us know how your dry run goes.