Its a blueberry M6411 with firewire, not sure of the MHz. I didn't know that changing the HD was so difficult, I thought it was going to be like my old black PowerBook?
He has just bought another one! I think its the same colour.
Then it's a 366MHz FW "indigo" with probably CD-ROM-drive (you'll have to take that into account, since for OSX Tiger installation you'll need either the CD-version or an external DVD-drive with USB or FW-connection or another (PPC)-Mac's in target-disk-mode and it's DVD-drive.
Replacing the HDD seems a bit difficult, but it's more a matter of patience and organization of all the screws and parts. Definitely no rocket science, but you have to follow the iFixit-steps and comments closely to avoid mixing-up things.
Dual-boot of os9+Tiger is a good combination, but you'll need additional RAM - best a 512MB-RAM-brick.
I'd go for a 120-240GB mSATA, especially, if it is to be used for store fotos, music and other stuff. Or maybe a virtual Machine with Windows98 or Windows2000.
I'm gonna post settings of my Clamshell later ...
PS: added the video-link about the fast HD-drive replacement.
Which black PowerBook do you have? A G3 I presume - maybe a Pismo with FireWire? That would be a great advantage, 'cause you can solve a lot of problems via TargetDiskMode. And with a PowerBook G3 you're a certainly a veteran of os9/early OSX.