You won't find many of those. There's nothing special about the model itself with its decimal tabulator, lack of a paper injector, and wide carriage. What makes it a less frequent find is the special order typeset. I'm sure Olympia sold a large number of these back in the day, but there are few of them that turn up for resale.

What I'm most curious about is its serial number. Looking at the machine, my guess would be that it's lower than 27,000.

If it's reasonably priced, I would jump on that SG! 

It is a marvelous machine, and because it's a standard they often sell for far less than portables, especially the trendy ones that some people pay a premium for. There are a few features that make the SG1 a better choice, but I'd still be pretty happy if a SG3 was my only option.

Off the top of my head:
Practical = four position ribbon control (SG3 only has three)
Comfort = spring loaded keytops (SG3 are rigid, like the vast majority of typewriters)
Aesthetics = subjective, but I prefer the more bulbous design of the '50s model, much as I prefer the look of a SM3 versus the SM9 (even though the SM9 is excellent to type with)
Laziness = external carriage release levers (they moved them under the case with the SG3)