the Arkel rack really is the best seatpost rack. If you keep your load light enough, you'll be fine, but I'd still recommend the Arkel, simply because it won't sway due to the design. They offer a backpack that slides onto the rack super-quickly and works well. If you're looking for a new pack as well, I'd consider the combo, but even if you are just looking for a rack, the Arkel will serve you well for light loads.
I agree with the others about the shoes. You can certainly carry them, but I think you'll find that they are the heaviest/most inconvenient/ bulkiest item for the bike commuter. If you can get a second pair to leave at the office, you'll make commuting by bike much easier. Same thing with the lock, if you can leave it attahced to the rack at work, that's onel ess thing you have to lug around/ potentially forget.