When going car shopping, try going in with the car seat and stroller you’re going to use, if you have it or can borrow from a friend. Based on my experience with the Mazda3 we had before our son was born I am gonna say the ATS is a non-starter. Fitting an adult in the back seat is not the same as being able to fit a rear facing seat and then being comfortable in the front seat. In the Mazda, my knees were wedged up against the dash, and I’m not a big guy (5’9”)
I get that fun to drive is still a factor, but the ability to fit (or not) the baby and related big-ticket items will outweigh any minute gains in steering feel or dash stroking abilities. That’s why in my mind, a GLI beats a 5dr GTI in daily usability. And I’m saying this as someone who’s looking to get one once Jr. is out of daycare/stroller/rear facing hell.
I’m very familiar with this car

and you are right, there are a lot of hours and trick work in some areas of the car like the C63 brakes; suspension is Bilstein club race coilovers and the control arms are standard pieces with spherical bushings). My favorite bit is the Mk60 standalone ABS system.
But all that aside, having driven a standard W203 as well, if you’re considering an E90 street car, a manual W203 should be a strong consideration given the similarities between the two. The W203 is a very underrated platform. If you find yourself straying from the manual-only requirement, the C350 Sport is 300 hp of reliable NA goodness. Not as good as an M-sport, but it won’t have any of the N54/N55 drawbacks.