$600 is a bit high, but not outrageously so. All functional pistols from quality manufacturers are somewhat overpriced these days.
Recent sale prices on GunBroker.com range from $625 - $716 for NEW or LNIB examples of the 620.
Sold prices have ranged from $499 - $638 (again, for NEW or LNIB examples). Of course, these prices did not include shipping or applicable taxes
Last year at this time, you could find a new 620 or 619 (fixed sight) for $550-$620. Used for a bit less...$450-$500.
Offer him $500-$525 and flash cash. If you really like the revolver, meet him in the middle. The speed loader and ammo are just a sweetner (and he probably has no use for them after selling the gun). On the other hand, at the recent price and scarcity of ammo, 3 x boxes of factory .357 and a speedloader adds about $60-90 of "boot" to the deal (depending on what brand of cartridge).
The 620 is a good .357. I had the 619, but eventually traded it away as I'm not enamored of current S&W production features and already have a stable of various Smiths.
My 619 was an excellent and accurate weapon. I'm sure the 620 would be even better due to the adjustable sights.
Hope this helps...