First essentials are original Win 98SE disk and boot floppy (i.e. Win98 startup floppy)
You'll also need all the disks for programs you want to re-install.
After that you'll have to download all Win98 updates and ditto for programs.
It's a big job, and if you use dial-up its gonna take a long time.
Might be better to use the opportunity to upgrade to XP?