I would invest in a helmet first. Then perhaps purchase an inexpensive computer (I say inexpensive b/c that's always something you can upgrade later once you determine the features that are important to you). I have the cateye mity 8 and it's just a simple 8 function, but that's all I wanted (cadence would be nice, but I can live w/o it).
I would definitely look into a clipless system, and that will make a significant difference. I have the ultegra pedals on my bike, and I have been very impressed with them. They aren't super light, but for the price you're paying, I believe them to be one of the best pedals available. As for shoes, go to the LBS and try them on. I tried on a lot of shoes, some fit me well, some didn't.
As for the saddle, measure how far your sit bones are apart, then search for a saddle which is slightly wider than the distance b/t your sitbones. Saddles you just have to try out, so keep trying them until you find one that fits you well.
Perhaps search the web for cycling clubs in your area, they are a great way to get introduced into the sport!