Do some shopping around. They can be had for a pretty decent price.

Out of the box, you shouldn't have to replace much. The stock wheelset is heavy but solid. I think the saddle is terribly uncomfortable but every butt is different and you may like it. Ride the pedals until you need new ones. The bars, in my opinion, are crap and should be replaced immediately. They're mock "track drops" that offer no room on the tops (unless you choke the stem) and the drop section isn't big enough for an adult-sized hand. The stock tires have really soft sidewalls but as long as you keep them up to pressure and aren't a 300 lb gorilla, they should be fine for a while.