Welcome to the forums!
First things first...I LOVE the C6 Vette! One of my favorite cars. I was lucky enough to drive one for a couple of days(the Regular C6 with the 6M, without the z51 suspension) and I have nothing but praise for it! It's a seriously underrated car in some circles IMO. Hopefully I'll be able to buy one someday!
It seems to me the MS3 would be a good fit for you, since it does have plenty of cargo capacity a nice interior and, for 2010, a slightly softer ride (mind you, most people suggest the car is better in the twisties for it). It's my DD and gets a few miles every day, but after a year and change of owning it, I have had no problems in any roads with regards to the suspension setup/height (and living here in Mexico, there's some pretty s*** roads). I don't encounter snow, but the 1st gen speeds (and I'm guessing the new ones) came with summer tires only, so a good set of winter tires is a must, but from what I've read here, that's about the only thing the car needs for snow.
As far as comfort in driving, I've done a few 600mi round trips and have no issues with noise, comfort or anything, hell I'm a few weeks away from doing a 1200mi trip, but some people do find the exhaust (again of the 1st gen) a bit loud for their tastes, so a nice test drive would be in order for you.
Regarding child seats, I have no experience, but do know it is compatible with them, since I got my car so it would last me a few years, I also wanted one that could handle kids.
The speed3s have so far proved pretty tough, safety wise, many owners here can probably attest (the platform was developed in Europe and underpinds the Euro Focus and the Volvo S40/C30, so it has a bit of sweedish blood in it), here's a video of an accident in a track of one.
Mazdaspeed 3 crash
The first gen's turbo engine has a powerband that is a bit narrow, kicking in at 3,000 RPM and then tapers off at 5,500-6000 RPMs, it is a bit pointless to rev the cars up to their 6500 redline; tough ECU programming was changed in the 2010s, I have yet to see a graph of it; but for daily driving there is enough juice under 3000 rpm IMO.
Reliablity-wise I've had a great experience (no problems whatsoever) but others have had some issues, tough most of them, with modded cars, these cars don't like to be modded for more power.
Hope this helps a bit!! DO go for a test drive!