Welcome! That's basically S&W's version of the Walther P990 (double action only). Striker-operated, no external safety, no decocker (since it's DAO). Looks like a good price. The Walther version are considered by most to be slightly superior in build quality, however S&W has great customer service and you'll spend twice as much on a brand new Walther.
I'm personally not big on DAO, but that's just my preference. The trigger pull on the 990 is long and heavy for every pull, but it's consistent. In other words, you won't have the long/heavy first pull, then short/light subsequent pulls. At least for me, a long and heavy DA trigger pull hurts my accuracy as I haven't developed a really smooth trigger pull yet. I also like SAO as every pull is consistent, but they're SA instead of DA.
My first handgun was a Walther P99c AS (anti-stress action) in .40, and it looks like that SW990 has the same dimensions as the P99c. I would go with 9mm if you can. The design, compact size, and polymer frame makes it very snappy in .40, but if you don't feel like 9mm is adequate for self defense, then by all means get the .40. 9mm will be cheaper to shoot and more controllable, especially when firing rapidly. But of course, the .40S&W has slightly superior ballistics.