We need to fix one thing. There is no such thing as a "G5 Mac Pro". There are "G5's"...and there are "Mac Pros"...but no "G5 Mac Pro's". Powermac G5's and Mac Pro's look VERY similar from the outside...but Powermac G5's are PPC based...and Mac Pro's are Intel-based.
This of course is pretty darn important stuff to know...if someone is thinking of buying.
It does sound like you are talking about buying a Powermac G5 (PPC based)...because these are the units that would be pretty "cheap" are are limited to OS 10.5 (Leopard).
As far as you purchasing one...they are pretty cheap of course...so the initial purchase is not so bad. But (to minimize costs after the purchase)...make sure the unit you get already has 10.5 (Leopard) already installed on it...otherwise you will be out another $75-$100 for a Leopard install disk. Also make sure it comes with a decent amount of ram. The Powermac G5's (depending on the model) either had a ram max. of 4gig or 8gig. If it were me...I would want to purchase a Powermac G5 that already had at least 4gig of ram.
Finally. Purchasing a Powermac G5 could be about curiosity or could be about purpose. If curiosity fine.
I have a Powermac G5...and I use it occasionally for some internet surfing (and e-mail if I wanted to). I also have a LOT of older software that I can run on it if I wished to. But someone else who does not have all this older software may be at a loss of what to do with the computer after playing with it for 15-30 minutes on the internet.
So I would say what would your purposes be for this computer? If curiosity...no biggie...they're cheap. But if purpose is more your goal...investigate the software you will need before buying (it may or may not be available).
HTH,
- Nick