+1 to mikeybikes thoughts on the matter
Everyone has different priorities. If you have a long commute and are for sure going to sweat because you want to haul butt. Cycling specific clothing will leave you FAR happier during the ride and at the end. I have biked 13 miles each way to work in many different types of attire, by far, bike shorts and either wool or cycling jerseys are the way to go. The cotton, which I have used, just flat out feels horrible when sweating into.
Panniers are going to be far more comfortable, but a normal backpack will do, unless you have thing you don't want to get wet, from your sweet and rain. At which point you would be happier with a nice water proof bag $$.
Oh, and try not to be wasteful and lazy, all you need is one extra tube. When you get a flat swap it out on the road. When you get 5 minutes at home or work, in a nice warm space, where you can relax and clean up, patch the tube you took out so you have a perfectly good replacement again. No need to go to the store between flats or stock piling tubes.
Enough with the chit chat, go ride!!
