"Go for a rucksack or large camelbak rather than panniers."
Really?! Have you actually tried a pannier(s) out? So much better, and more comfortable. Less strain on your back, keeps you cooler, easier to carry heavier loads if you can etc.
If you can change at work then mudguards are less essential, but it's amazing how often you get wet from road spray rather than rain, so mudguards make a bigger difference than you think. And it's all about full-length one.
Also no-one's mentioned the benefits – I find that a commute of that sort of length makes a big difference to fitness – it'll be hard the first few weeks, but when you get into it then it helps a lot.
Also eat more breakfast, and make sure you have something healthy to snack on when you reach the office – commuting makes you hungry!