After some searching on the web, I have discovered that buying a new original 10GB replacement for the broken hard disk drive would cost me more than the laptop itself is now worth, as I suspected. This is not an option.
But having taken a closer look at the “caddy” containing the drive, a newer drive should fit. It will be thinner than this one, but as long as the corner holes for the screws are in the same place, it should be fine.
I have also searched for info on hard drive capacity limitations. I am now almost 100% sure that the laptop can utilize hard drives with a capacity up to 137GB. Since the 40GB drive I have selected falls well within this limit, it looks like all possible obstacles preventing me from buying this thing are gone.
The day after tomorrow, I shall purchase the new drive and see how it goes.