Reference your question as to worth of M-51, my opinion is yes since in this area they bring closer to $500.00 to $550.00 especially if the person making the purchase has been looking for an extended length time. If you can afford to shoot the Magnum Ammo then you will have a very nice revolver. .02 cents worth or less!
You will love your 51. Great little backpacking piece. I always wondered why S&W came up with 3 1/2" as the only barrel. Now a 6" would really be something. Like a Model 35, which is another of my favorites. $500 is a go as you don't see many for sale and even have a chance to dicker over.
All the Best,