As has been noted already, it doesnt take much to keep a Smith running for a long lifetime of use. I would suggest a mainspring (I've had exactly 1 break in all the Smiths I've had over the years), a hammer nose (Smiths name for the hammer mounted firing pin) and hammer nose rivet, and an oversize hand, if they're still available, if not, then a standard hand. I would have a rear sight blade w/ screw and nut, and an extractor rod. A spare thumbpiece and thumbpiece nut arent a bad idea either. They can become loose and get lost. Spare sideplate screws arent a bad idea, and the front screw for the rear sight assembly is small and tends to loosen sometimes.
At the bare minimun, I'd have a mainspring and a hand.
I've never broken a firing pin, but it sounds like someone else has. I've replaced a hand or two, over many years and many many rounds. I've broken a rear sight blade from dropping a Smith.
Brownells is a good place to get Smith parts. I believe their web site has schematics for the various models.