I'd at least pull apart the original pump and check it out- if it looks good, reuse it. I've read some horror stories on here about brand new pumps still having machining debris in them (not my personal experience, but forewarned is forearmed), so I'd be pulling it apart anyways just to check. Just make sure the pickup tube and screen is nice and clean... (have them throw it in the hot tank with your block, if that's the route you choose to go).