hard drive does not boot but is readable

Ah, you got it fixed, the HD is not shot, the OS on it is most likely corrupted or the permissions were wrong, for instance, if the permissions on the root level is set to be executed by nobody the system will refuse to boot since it has no permission to do so.

 

I've got a Powerbook G4 1.67 (And a new 2012 Min), with 10.5.8, it still runs smooth as long as I don't use Flash for instance, this was the last OS it could run, SL was even faster since they got rid of all the PPC code.
The Mini runs fast as well, except the ridiculous slow startup and shut down times, my 8 years old PB does that much faster than the new Mini.
Yeah, you could try to install SL on it, you can do this by starting from the installer DVD and then choose the external, I know a trick which can do this without running from the DVD but is a bit complicated, you need Pacifist (App) with this and then open the OSInstall.pkg which is in the /system/Packages folder on the DVD.

 

Depends on the Model, I asked you yesterday but you never told me, if you want to find out yourself there are several ways, a nice free App with all specs of most if not all HW and some software is MacTracker, around 50 MB download, another way to check is to go to everymac.com.
The App shows you what the original OS is and what it can run amongst much more info, try it.
The App itself is (a bit) poorly written, sometimes it does not open, sometimes it crashes and it's slow to load, but on your rMBP it should be much faster than on my HD Mini.

 

Cheap, make sure it's the right part, and if you fix it all you will make quite a bit of money on it.