The $500 price range will probably limit you to relatively new S&W M29s and M629s- mid 1980s and earlier. Due to collector interest, and relatively limited production prior to the mid-1970s, earlier .44Mag 'Smiths will generally be priced out of your range unless the cosmetics are poor. However, there's generally no reason to prefer an older model over a newer one if you're after a "shooter".
S&W revolver engineering changes are designated by a dash number stamped inside the yoke cut, e.g. "M29-3" = Model 29 Dash 3 or the 3rd engineering change. Initial versions are known as "no-dash" guns.
Pre-1987 .44Mag Smiths had some problems with lockwork "shooting loose" from recoil when subjected to heavy use with full-power Magnum loads using heavy bullets. S&W cooked up improved lockwork, titled it the "Endurance Package", and installed it in all Model 29s and 629s starting with the 29-3E ("E" for Endurance") and 629-1E. These revolvers have gone through several engineering revisions and successive decades of production since then, all of which have the improved mechanism.
However, the early guns do fine the way most people use them- a diet of mostly .44Spl with a few Magnum loads now and then. This is good, because there are a LOT of M29-2's and M29-3's out there; most of the "shooter grade" M29's I've seen are these versions. Most of the newer ones just haven't been around long enough to accumulate a whole lot of wear.
The 1990s "Classic" and "Classic DX" models feature full-underlug barrels like Bob Wright's gun; these are very well-liked by .44Mag enthusiasts because the heavy barrel mitigates muzzle flip, particularly on short-barreled guns. (Don't confuse the 1990s "Classic" with the post-2009 "Classic"; the latter is designed to look like an older model with a standard barrel, just with "The Lock". The older Classic has a full-underlug barrel that actually says "Classic" on it.)
On the flip side is the "Mountain Gun", which has a lightweight carry-friendly 4" tapered barrel. I've not had the privilege of setting off a Magnum load from one of these guns, but I've been assured it is not the most pleasant experience.
I recommend sticking with the longer, heavier barrels for lots of Magnum shooting. (OTOH the Mountain Gun should make a great platform for launching Specials.)
IIRC models prior to the M29-8 and M629-6 lack the S&W ILS "Infernal" Locking System that many revolver enthusiasts love to hate.
However, the lock can be removed by following instructions you can find in numerous places on the 'Net.
That should get you started.
