You don't need it that much. If you have an external SCSI hard drive or a Zip you can use those. They're much faster anyway. The PB100s are the only PBs to use lead-acid batteries. Consequently, although these give reasonable performance, they do not have a particularly long life and can be harder to find.
Spares are highly advisable.
Try turning off the backlighting. In the old days, enterprising companies made PowerBook lights which clipped onto the PB screen and shone a light into the display. Most desk lamps do an equally good job. You may occasionally find these at parts shops. But I don't recommend it unless ethernet is essential as it just hogs your convenient SCSI port.