I have to ask when buying used are there any certain models I should stay away from, such as defects, weakness, etc....
That is a gigantic question to answer.
Defects......no
Weaknesses.....of course, however most are corrected when learned about. For example if you wish to shoot lots of heavy 44 Magnums you want to stay away from the model 29s manufactured before the Endurance package was added to the firearm.
If you plan on putting a scope on one you want to choose an engineering revision after the frames were drilled and tapped for scope mounts.
If you like shooting 45LC with the original .454 projectiles you need to get one that was made before they switched to the tighter throats for compatibility with 45ACP projectiles.
Some traditionalists refuse to buy S&Ws after they stopped making P&R guns, others refuse to buy after wood grips were discontinued, others only want forged hammers and triggers, after all they don't build them the way they used to. While those that embrace advancement like the CAD/CNC firearms for their greater strength and are willing to overlook the artistic beauty of the old blueing that the EPA took away from us.
Some embrace the extraordinary accuracy that the multi-piece barrels have brought to the revolvers in recent years, rivaling the legendary performance of the older Dan Wessons. Other will tell you it is a way to cut costs and just make the gun cheaper and they want nothing to do with it.
Some love plastic guns because they weigh less other prefers blue steel and Ivory for their automatics.
There are hundreds of little idiosyncrasies and tidbits. If you do not already have one, a copy of the Standard Catalog of Smith and Wesson (SCSW) will tell you most of this stuff. The 4th Edition will be releasing within the next 6-12 months if you want to wait.
If you want to discuss it here, at least narrow down what you like to less than "Everything S&W"