Look for a date code (two letters) probably under the front of the slide. A=0 B=1 C=2 etc. That will let you know how old it is.
Is it a standard aluminum model or all stainless?
I wouldn't say there's really any past models to avoid. A lot of people complain about the newer Sigs. I prefer the old W. German models.
The main things to check on a used Sig are the rails and the barrel. If the Sig is actually in mint condition, there shouldn't be any problems.
Heckler & Koch Club Member #26
"What about learning to use a gun?"
"Oh, Johnny didn't believe in guns, he always thought..."
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