Bro there are parts which you can easily judge from their appearance, scrap yard is okay for these, but in the case of fuel pump, how to get idea of motor's condition? Who knows how much wear and tear it has already taken? It is all luck. Might get a good one but I won't go for that. New one is not that costly.
Lower pressure is a problem but higher is not. All of the coming fuel is not forcefully injected into injectors. That's why there's a regulator and a return line.