How do I pin point a start up issue?

Hi, I was sold a really bad g4 power book and now I want to see if I can fix it. It seem like the seller just put a bunch of failing parts in a 01 g4 and lied and said it was a 03. Anyway here is a list of issues

The fan is not working

The cdrom wont read any cds just ejects the after a few moments

The battery does not old a good charge

The screen is getting dimmer

It only boots to the flashing question mark folder

will boot open firmware and option menu but no other commands.

I need to hit the little reset button in the back each time I want to turn it on, it will boot and chime but if I don't hit that lil button there will be no display on the screen

The seller lied and said it had a 40 gb hard drive but it only has a 10 gb drive and it is not held in place by anything. Also its not the original HDD that came with this g4 as the battery panel sticker says it came with a 20 gb HDD.

How can I tell if its the logic board, pmu, pram battery, inverter board or what ever that is causing multiple component issues or if the thing is just full of bad parts?