First, what Mac OS are you currently running on your Power Mac G5? And associated with that, what third party software are you using?
Secondly, differing hard drives has no effect on the Mac OS, applications, etc. The more important issue is how old the hard drive/drives is/are inside the Mac Pro you want to purchase, and if they have been taken care of. That last part could be difficult to determine. (You also might want to consider if such a Mac Pro you want to buy has an SSD/SSDs inside).
Third, how much computing "power" do you need? Myself, I used to have a Mac Pro machine, but eventually it was no longer supported with the more "modern" versions of the Mac OS. So, for my needs, a Mac Mini was perfect, and I purchased one about 4 years ago (it is a late 2012 model). I was able to just just about all my peripherals with it (monitor, keyboard, external hard drive, etc.). While not a real "powerful" machine, it easily serves my purposes.