A new one, right? Not a used one? This will probably help:
https://www.apple.com/powermac/specs.html
But to quickly run through some answers:
1) The SATA drives will be fine, and yes you’ll have to wipe them. You can only fit 2 drives in the case, however, which means you can only add one more. (There are people out there who sell solutions to cramming more in there…but I’d rather just buy an external enclosure and a PCI SATA card.)
2) Given the tightness of the case I wouldn’t worry about any other drives. (I’m pretty sure they won’t work anyway, but I’m not even going to check since you’re out of space.)
3) PCI-Express for the graphics card although I’m nearly certain that video cards are still sold as either PC or Mac versions. So you won’t be able to take anything with you here. At least it was that way with my G5, although it’s an older model. This is something to ask someone at the Apple store if you can as I can’t say for 100% that I’m right. Although if you have an AGP card, again like #2, I guess it doesn’t really matter.