If you could repair it yourself, you'd probably know you could.
I'm guessing you have a (20"?) G5 iSight? I'm not aware of any REP on the iSight models. If you are referring to logic board/power supply REP, that was for the G5s without iSights, and if yours is 18 months old, it must have been one of the last few G5s of all so your serial should be a way out of the affected range for that one. Not that I'm helping.
If what I have said rings true, I can only offer advice on dealing with Apple. If they have repeatedly refused to cover it under REP, then someone has been briefed not to overstep the bounds of the serial range at all. You should abandon the REP and simply suggest that owning a machine for only 18 months and having it rendered beyond economic repair is unacceptable to you. Point out how much you paid for it, how long your previous Macs have lasted (back in the day they would go for years), and compare prices and warrantys with other manufacturers.
If you still aren't getting anywhere, your last resort should be to offer to pay for Applecare if they will agree to cover this repair and give you the other 18 months on it. I have known them to make this deal on occasion. But don't stop at the first rep. Make the same call more than once if you have to. Some reps know the sytsem better than others, and some are more willing to go out of their way for you. If you can get hold of a customer relations agent, they are the ones who have the power to authorise such things. They are usually well protected by a screen of applecare monkeys however.
You could always try crying down the phone at them.
By REP I mean Repair Extension Program. By rep I mean representative.