Well, I just had an iBook G4 come across my workbench today, and it was a right PITA to get the thing apart. The hard part is getting the bottom case separated from the rest of the system and not bending things up badly. And things ALWAYS look easy when someone else is doing it.
If you don't care how it looks after, just so long as it works, have at it. ifixit has a couple of guides. Right now it doesn't work at all, so you've next to nothing to lose by trying, but a system that old could have a large number of problems. It wouldn't surprise me at all if one or several capacitors had simply gone bad. A system not starting regularly is actually a pretty common sign of a capacitor problem, though not conclusive by any stretch.
Just set a hard limit on how much money you're willing to risk on this system, and set it pretty low. I don't know what the conversion rates are between US and AU dollars, but even if that system were in perfect working order, it probably wouldn't be worth more than $250US. Even a used logic board off eBay is probably going to be $300US, and you have to be VERY sure that it's going to work with your particular system, and you never know if the other person has any idea about ESD and proper handling of sensitive electronics. So, figure out how much money you can afford to potentially throw down the crapper on this system if things don't work after you're done, and don't exceed that amount.
Personally, I would think a brand new MacBook would be looking pretty tempting in comparison. Whatever money you're willing to potentially waste on this old clunker could be put towards a new system with almost infinitely better specs. A new system would probably be at least two generations beyond what you currently have, maybe three. You wouldn't know what to do with all the extra time you have now that you don't have to wait for everything to load like on the iBook.