By 1989, the 629-3 was out - with the 'Endurance Package', which included the heat-treated (hardened) yoke, among other things. The lengthened bolt stop notch and bolt were also added by then, enhancing durability. I feel that the MIM parts and frame-mounted firing were also great 'shgooter' enhancements. With this in mind, the 629-6 seems optimal for a shooter. I bought both new - a 4" (#163603) and a 6" (#163606). They are 'current stock' - and, along with a Wolff 'standard' hammer leaf and the S&W Accessories .500 Magnum Hogue monogrip, will likely cost you less than the 629 at $750 which you mentioned - and be a far more capable, and kinder recoiling, example. Plus, bought new, you'll have that lifetime warranty at your disposal, too.
If you are a collector - and that 629 is complete - it may just be worth it. Not to me... I like to shoot them! I popped through the local steel plate challenge plates at the range yesterday with a hundred of my .44 Russian homebrews in that 4"-er - really fun. Of course, the .500 Magnum Hogues really ate up the massive recoil, TIC, from those mighty 240gr LRNFPs - making ~720 fps... Really, they can be fun with Russians and Specials, too - just clean those chambers before you get out the Magnums!
Stainz