You should use both to satisfy the fans which are familiar with the series. If you can create new art that's even better! Don't worry about copyright, it's a MOD, right?
I totally agree with Gfire. SC2 will be worth the money spent simple because its editor will be a powerful tool which will allow moders to create great modifications. Imagine for example DotA in SC2 editor! There are even rumors that the editor could be used to create FPS mods.