Sitting in a pack, a sub-5 century is almost easy. In general, you want to have the miles in your legs, and the ability to cruise for long comfortable periods in the 20+mph range.
Any quality road bike will be fine - as someone said, the main thing is to be comfy and efficient on the bike. As you train, you should also be shaking out mechanical and position issues, so that by early / midsummer the bike is bulletproof reliable and comfy for hours of steady riding.
If you want to upgrade, best bang for your buck is new, lighter wheels.
If you're planning on using a pack / drafting, don't bother with aero bars. If you expect long periods of solo time, get inexpensive bolt-ons. And train with them - they require getting used to.
You need a training plan. I would suggest at minimum hitting the local hammerfest ride(s) every week, and as the season warms up into Summer, tacking on additional miles before / after so that you can knock out at least 85-90 and finish feeling tired but still holding your target pace.
Food and hydration are critical. A camelback full of your go-go juice of choice, bananas, 2 big bottles, and a ziploc of more powder might be enough. This will minimize rest stops. You need to be training in this mode all Spring / Summer so you have all the bugs worked out.
Do speedwork. "You have to go fast, to go fast." Look up the threads on 2x20's and do those 1 or 2x a week.
Last edited by Creakyknees; 11-07-08 at 10:24 AM.