An interesting project...
The down side of (some) iBook G3s is the dreaded logic board problem. My suggestion is to get an iBook (min. 700MHz, 384MB, QE for OS X) that is covered by
the replacement program. Then try to get Apple to swap the logic board, just to be sure it does not give up on you on the road. Oh yeah, also remember that the G3 iBooks have tray loading optical drives that could break up easier than a slot loading (the G4s).
For a tutorial on opening the Dual USB, check
this site. As for exterior improvements... In my opinion the casing is pretty well fit. I suppose it could not hurt to improve the seal with silicon paste between the edges of the top and bottom case if you're taking the machine apart anyway. One thing that are still unprotected: the connectors on the left (ethernet, firewire, etc.). I'm not sure if anyone makes covers for those... Finally some information on
painting the case.
Good luck and report back
