1) Only buy a new bike seat if your current one is hurting you. When you go buy one, the salesperson can help you pick out what works best for you. Now, DO NOT go buy a new seat right before the event. Give yourself at least 2-4 weeks in the new seat to get use to it.
2) For triathlons, we wear tri specific shorts. They are made to go from the swim to the bike and the run... that way, you don't have to worry about changing clothes in your transition. Tri shorts, coupled with a tri top, are a wonderfuly investment. Only wear them racing and they will last longer. Plus, the padding in them is so minimal, it's not best to do long distances (over 25 miles) in them.
3) Good luck! Make sure you come back with a race report!
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"