I have never found that flushing a system made much of a difference, but it wont hurt anything. If your system didnt have enough antifreeze in it, there would be more boiling after you had shut it down; compared with the amount of boiling if the coolant was a 50% mixture of water and antifreeze. Lots of time with a leak, only water is added, and the percentage of ethylene glycol decreases as only water is added. That would be more likely if the prior owner had plans to sell the vehicle anyway.
If the radiator is original, or hasnt been changed in ten years or so - then a new radiator wont be a waste of money.
I agree with doing a pressure test on the system too.