In a small bowl, whisk together the mayonaisse, mustard, vinegar, sugar, celery seed, salt and pepper. Set aside. In a large bowl, stir the diced potatoes, chopped celery , onion and hard boiled eggs together. Add the mayonaisse mixture to the vegetable mixture. Gently stir all the ingredients together. Refrigerate for at for least 1 hour before serving. Sprinkle with the paprika just before serving.