-Must be Windows XP and have original install disks as well
-Must have Wifi and relatively good working battery
No more than $350 (before shipping)
I've been watching eBay for something similar for awhile, for a friend who can't make up his mind if he wants one or not.
If you're willing to give up the "must have wifi" requirement, there are usually several older IBM ThinkPads that fit that description. Wireless cards can be bought for $20 or even less when they're on sale at a place like CompUSA. I wouldn't pass up a good computer that you like other than it not having wireless when you can get a wireless card relatively cheaply.
Another thing I'll point out... quite often new computers don't even ship with install disks anymore. All the install files are on a service partition on the hard drive. I would suggest that an intact service partition is just as good as having physical disks, but if you don't feel the same, make sure the seller is clear on what install media you're getting.