I am in need of lots of guidance (essentially start to finish)

Hello everyone! I have never done a mackintosh nor built my own PC before. I've heard its easy but I just have never done it before and I don't have the disposable income to mess up.

I have a PowerMac G5 that still works with the case, the Apple Studio Display, keyboard and Pro mouse. Just turns out Mac OS X Tiger/Leopard just don't cut it anymore though

Ive got a pretty low budget of at least $500. I don't expect to get this done for a few months but Id like to know what to buy and what to do and watch out for.

Id also like to keep it as stock as possible with the hardware and be able to use the front power button/ports

Thanks!