I'm highly thinking about buying this labtop and updating it to ios7 and rebuilding it . Is this a good idea or should I just save and get the new MacBook Pro
iOS is for mobile devices (e.g. iPhone, iPad), not for Macs. Also, the PowerBook (and all PowerPC Macs - G3, G4, etc.) cannot run any OS newer the Leopard (OS X 10.5). I hope this helps.
I'll just suggest steer clear. I had the 15 inch a little while back, while it is a capable albeit slower machine its frustrating in that it leaves you wanting more. Not to mention due to age and availability of components upgrades are prohibitively expensive.
The Powerbook G4 is a dinosaur that has been obsolete for quite some time...unless you just have a nostalgic connection to this thing, I'd save your pennies.