I wouldn't get a g5 now.
Much slower, older graphics card, and not very future proof.
I'd never buy a computer off eBay. Too risky.
I recommend getting a refurbished iMac from Apple. A little bit more expensive, but comes with one year warranty. That piece of mind and reliability is worth it.
Just don't buy the overpriced Apple RAM. Get crucial RAM (sometimes the refurbished computers have much more RAM in them anyway, so you might not need to buy additional ram).