Can't stress to remove the battery thing enough!! That'll eventually leak and eat right through your motherboard, damaging internal layers which are impossible to repair. It's possible to do some modifications to make it use a CR2032 battery (the type used in most modern machines). CD drive should be no trouble at all with IDEFix 97 as someone said, and any IDE cable will do.
After that, be careful of using hard drives bigger than your current one, as your OS version (3.0) doesn't support them. Updating the scsi.device and using a replacement filesystem such as SFS will sort that out, just ask here again if you do go that route

If you're replacing it now, you might as well get a brand new 80 or 120GB drive as these seem to be the smallest available, and are cheap as buttons.