Hi,
First of all should I get it?
You will have to decide that, but the PowerBook 540c is an excellent intermediary between old and new technology. Please note that the battery probably is bad, so do not expect to use the computer for longer periods without an AC adapter connected.
Can the hard drive be upgraded? Can it be replaced if it fails? Where do I look for old SCSI HDD for it?
The maximum size for a SCSI 2.5" hard drive is around 1 GB. If necessary, it is not too difficult to replace the drive. Working 2.5" SCSI hard drives are relatively rare these days. You could perhaps try an online auction site.
How do I take files off and transfer to my iMac? Network?
If the "network cable" is the required external AAUI to RJ-45 transceiver, you could connect the PB 540c to an Ethernet network (or directly to another computer).
If the PowerBook is equipped with the (optional) PCMCIA Expansion Module, you could use a CompactFlash card in an inexpensive PC Card adapter for CompactFlash. A USB card reader would have to be connected to the iMac.
You could also connect a USB floppy drive to the iMac.
Finally, if the iMac can burn CDs, the SCSI CD-ROM drive should allow you to read discs at the PB 540c. Use CD-R, not CD-RW.
Jan