It should detract slightly, if they can produce the paperwork and that it was an actual new engine, not a used motor. Id want proof of the labor, and the parts purchased and replaced. My 05 had 15k at engine replacement and the local subaru shop did all of the work and the current owner of my old 05 can supply each and every receipt of that job (no it wasnt warrantied). Thats what I would look for. If they can provide the necessary paper work Id take $250-500 off of your offer or what its worth and go with that.