To decide if a spare pump is required you can ask yourself, "If this pump were to fail, how long would it take to repair/replace and at what loss?"
If this pump is critical then you definitely need a spare.
Another question that I pose along the lines of spare pumps is how often...
As everyone has indicated you must first have an accurate simulation. This will take a large chunk of time, but will payoff numerous times over. Your preflash drum (or tower if you choose to fractionate) won't necasserily be at the end of your preheat train. To determine where in the preheat...