WE PMC AK would be modeled after an AKM of some sort, the 7.62 mags and slant flash hider gives it away. The AK74 shoots 5.45 and uses the plastic 5.45 mags, has a different flash hider, and various other minor differences. Though for the sake of airsoft they're pretty much the same sort of looks. Use 5.45 mags and swap the flash hider and you can call it an AK74 and most people won't be able to tell the difference.
As for the rest, someone else can probably give you the answers or you could take a look on the gasgun forums, lots of good info there.
Just your friendly neighborhood narcissist gearwhore.