Answering you a bunch here but in saying this I don't claim to be an authority on the subject.
Any of the Sig DA/SA can be outfitted with it. Might be $150. However, based on Sig store feedback in a Legion you get both a hand fitted and SRT - a nicer trigger versus just short reset. And upgraded sights, all sorts of stuff. If you really want and would be happy plane Jane 220, they are out there. If you are gonna upgrade what you got a bunch - I'd say probably makes sense to get a Legion if you like it as is too.
As far as triggers for instance I have a 226 tacops, it came with a SRT but reportedly not a true Legion quality job. Honestly I can't tell. But my impression is if it's not one of those $1200+ models, usually they lack a SRT. I have a 229 Elite, some day may have it converted but it is not an SRT.
Now as far as SRT goes, it's nice, and someone will probably lose their shit over this, but I will say could be more likely to cause an accidental discharge (in the direction of what you were discharging at already). A light trigger that resets with a tiny bit of travel and your finger on the trigger from the last shot - you get tired, beatup, laxidazical, the likelihood goes up. I usually shoot in a pit with a huge berm, typically alone, very controlled as far as margins of safety, and at times do stuff that I wouldn't ever in a range or without such a margin of safety - especially as far as not taking a break when I should. But it only has happened with those DA/SA SRT Sigs.