Plenty of people do it every year, so it's possible. Like others have said though, it's never a garauntee. I managed 1000 miles before injury forced me off the trail.

As for what piece of gear to spend the $$$ on? Hmmmm... I'd say the sleeping bag. Get the best, lightest down bag you can find. The cost, spread out over the life of the bag, will be cheaper than even a really good synthetic bag anyway, as down bags tend to last longer.

Look at Western Mountaineering bags, or Feathered Friends bags. Both are top notch.

And trail runners vs. boots. I would say start now getting your ankles into shape and go with trail runners, especially through the desert section. It's hot in the desert, and runners will be more comfortable.