Choose a fork -- suspension forks cost ~$200 to start, decent ones, anyway. A rigid steel fork can be had for half that. You want some cush up front?

Headset with cartridge bearings can run as low as $15. Handlebars and stems can each go from $20 to over $100.

A wheelset can be as low as $100, or as high as $1000; good ones are available at the low end, I have a Deore disc/Sun Rhyno Lite wheelset, ran me $110 shipped.

I prefer SRAM shifting; X.7 setups are ~$100.

See if you can find a Deore Octalink crank, the bottom bracket (crank axle assembly) is still available, reliable, and cheap. Got an ES-25 last winter for $5.

Go Avid for brakes; FR-5 levers are nice, and can work with either the mechanical discs (BB7) or the V-brakes, which are excellent.

ODI or Lizard Skins lock-on grips, in a variety of thicknesses -- pick one.

Generic aluminum seatpost that fits, saddles are a personal choice -- try some out at the LBS.