I'd say go ahead, it seems like a fun project to you so it's all cool. Yeah I recommend PPC forums as well.
As for cooling, luckily you dont have liquid G5 so I think it should be possible to cool it fairly easily. You say you have big-ass tower so that's good, that means you can mount lots of fans on the case. That might help you overcome the airflow issue.
The only thing is, you cannot plug in the fans to G5 motherboard, it won't probably have the correct power pins but I bet there's a way around that.
Anyway... you probably need a second power supply anyway to drive all those drives, G5 has only power cables for 4 devices if I am not mistaken. There is probably an adapter to power the fans, I haven't built a PC in few years so more experienced people will probably help.
If you post a new thread in PPC forums, can you please link to it? I'd like to follow your progress.
Also, once you get more $$$, look for used Mac mini. They're pretty cool file servers, I have 2009 C2D model with only 2GB of RAM but it works great. It serves as a Plex multimedia servers and is able to stream simultaneously to multoiple computers. I have Snow Leopard running on it.
I still think that very elegant solution would be to use Mac mini as your server and hook the drives via FW800 enclosure but youre right, they're damn expensive and if you add Mac mini to the mix its probably not worth it. I see big advantage in the low power consumption of Mac mini as opposed to G5, if it's going to be used for most of the year, your electric bill might be a bit high so who knows whats financially more sensible
This is actually what I want to do with my Mac mini once I get a good price on FW800 enclosure. I'd like to have at least 2 drive FW800 enclosure, but 4 drive would be better. I don't want NAS.