Answer to "My iBook is not booting, full speed fan, black screen"A bit late to answer, but your issue looks like the dry solder issue iBook G4 was plagued with. And yes it can be fixed: https://macintoshhowto.com/hardware/how-do-i-get-my-broken-g4-ibook-fixed.html
Answer to "Mac with most repairability (10 out of 10)?"The Macintosh Portable is the easiest to open and access it's internal parts. On the desktop line, some modeles like the PowerMac 7500 is really easy to open too.