Not to poopoo on your plan, but just wanted to give you a heads-up.
If you're looking to get back into the hobby and get excited about it, piranhas are pretty boring fish in my experience. I've heard that they can be cool in a big group in a huge tank, but I've only watched a small group (4 +/-) in a 55/75. They're really good at hiding when they see you and despite trying really hard (even built a mini deer blind out of chairs and blankets in front of my buddy's tank) to watch them eat, never saw anything remotely worth the hype.
For excitement, I say get some Africans or some bigger central Americans.