I think it's a pretty good price, and it's certainly a great car, but it's not the car I would buy in your situation. As others have mentioned, you're financing this thing for
six years. It was a brand new, first-generation car in 2013, and while far from assured, there could be problems with long-term durability that we won't even know about until the car has a few more years on it. I don't think I'd be comfortable with only 7k miles on the factory warranty in a situation like that. If you can afford an extra $1200 or so, I'd at least try to get a CPO warranty on the thing (I think you can buy them after the fact from Cadillac dealers who offer them), just for peace of mind.
But honestly, if I were in your shoes, and I wanted a quality car for around the same monthly payment, I'd be looking for something at around $20k, financed for 5 years or less, with a loan from a credit union that should run around 2.0%. So, what's a good choice at that price point?
That runs the whole gamut, from small and sporty (FoST) to big and luxurious (Genesis). They're all within your budget (and cheaper than the ATS). They all have lower miles with tons of warranty remaining. They all have navigation and a sunroof (I think only the Focus lacks leather and heated seats). They're all, in my opinion, a better buy than the ATS you're looking at.
Not to take anything away from the Cadillac. It's a great car (I myself drive a 2.0T Performance model with the same black diamond paint, nav, cold weather, and sunroof). And at every price point (from factory new down to this used one in good shape), there's unquestionable value compared to its typical German competitors. But personally, I just wouldn't want to own it out of warranty within the first year of purchase, knowing that I have 5+ more years of payments to make on it, should any issues pop up, especially on a limited budget.
Forgive me if I went a little beyond what you were looking for. I spend a lot of time on
/r/whatcarshouldIbuy, so I automatically go into that mode when people ask questions like this. Good luck! If you do buy the ATS, I'm sure you'll love it. I got the Performance pretty much just for the lights (and among the cars that were on the lot, it didn't add much to the price). I tell people that if I were to do it again, I would have gotten a Premium model without the added expense of the sunroof and sparkle paint (each one was a $1,000+ option), which would have given me MRC and HUD and 18" wheels for about the same price I paid. But you know...you have to draw the budget line somewhere, and I'm happy every time I see and get in the car.
edit: I just noticed that you said it would be $23k including tax and title, which knocks down my comparable prices a bit, but I think all of those cars can still be had close to the lower price. But that ATS is about 50% of its original MSRP just a year and a half ago, which is hard to argue with. So yeah, it's a good deal on a good car. But still, the previous owner also knocked out 85% of the bumper-to-bumper warranty by putting so many miles on it in that same year-and-a-half. If it were 24-year-old me, I would probably get the Focus ST (incidentally, 24-year-old me
did buy a 2004 Jetta GLI). And 33-year-old me would get the Genesis if I couldn't afford a newer ATS.