+1 what DarkSoldier said....considering the 3rd gen S&W autos, I'd look at the 9mm versions first and the .45acp versions second as these would have been stressed the least in firing - given equal firing time as the .40 - in other words, the .40s would have been stressed the most out of these three calibers....and if .40 was all that was available in the used S&Ws, then I'd look at another brand...used Glocks are great, I prefer the 9mm in these as well and if anything is needed or wanted, you can do it yourself, they're simple and durable - like a good model 64 S&W .38 special revolver only it holds more ammo!
Yep, you can't hardly go wrong with a good 'ol model 10,13,15,19,64,65,or 66 S&W revolver, that is, as long as the magnum versions haven't been fed an extreme amount of magnum ammo and stretched the frames out...buying used in the .38 special only line (10, 15, 64) is always a safer bet as to getting a good used S&W revolver ...
The GP100 line of Ruger revolvers are excellent values as well as their P series autos...you can usually buy these new for what a used Smith or Glock will cost...(I just prefer the triggers on Smiths and Glocks)...
Let us know what you decide....