tub and shower trim

I have this Old Shower Kohler handle and trim plate that I would like to replace. It operates just fine, just want to replace the trim and handle for aesthetic purposes. I have already seen a video of how to remove old one, seems very simple. But that vidoe only showed them repairing it, not...

I bought a used shower trim, i know its a Pegasus but there is no other identification on it. I am building a new house so if you could tell me what i would need to buy to make this trim work? I dont know what i am doing.. I think i need a cartridge and a valve? Anything else? and what kinds?

Please help to identify this tub and shower trim kit