SIG's quality control went all to hell iin 2006-2008, when they discovered MiM and boutique finishes and forgot about decent parts and tolerances. Anything older than that (easy way to tell: too old to have rails) should run like a raped ape. Anything newer that, I'd be a little uncomfortable.
If I were looking at a railed SIG, I'd want a test-fire and inspection. And if it passed both of those, I'd also just make my peace with the fact that, wherever the manual says "change this spring/pin/etc at x000 rounds," you'll probably need to knock a third off of that round count for full reliability.
Between the pre-rail P226 variants, I don't have a strong opinion. I love them all equally (although my old P228 is a jealous bitch and pokes me right in the right kidney whenever I look at another gun)