Swapping in a long block into my 2000 Subaru Impreza RS?

My uncle had this 2000 Subaru impreza RS and he bought a brand new wrx. He didn't know what to do with it so he gave it to me. I want to put a new engine into it and i want to go with a long block because i don't want to deal with the requirements that a short block needs. I am wondering what kind of engine, what year engine, and i am trying to go with either a wrx or sti engine. What would be required besides a wiring harness and a new ecu ? If you could answer all my questions and give me some advice and more information. I would much appreciate it!