Welcome. You can expect to get a lot of miles from a Mirage. Way more than your Fusion and a far lower cost of ownership.
What mileage you can expect will vary on you and the traffic I guess. I'd expect you should be able to get 40mpg in 5spd doing 75mph, others will chime in... I average about 40 something mpg without really trying hard in my 14 cvt model. The drivetrain really didn't change much over the years, the 17' got a few more heresepowers otherwise most parts are the same(other than brakes and exhaust).
As far as winter goes, we get winter Alberta. These cars do fine in the snow and ice. I had a Caravan with blizzaks one winter and it sucked compared to my Mirage with cheep china winter tires that cost $40something a piece.
I maintain a fleet of 30 something 2015-2019's and they hold up great. I've never replaced an engine and none of them have any leaks. Some have well over 300k kms with one almost 480,000kms(replaced cvt around 300k km).
There is a thread on here about someone that traded in a Mirage that had way more miles on it.
Most people are pretty happy with these cars, I bought a used one a year and a half ago and recently sold it for more than I paid. And I picked up another one from a member on here a few months back with a wonky cvt . It's got 286,xxxkms on it now. All I did to it so far was front brakes and a windshield.
Most car reviewers hate these cars. They don't drive as nice as a toyota or honda but they're cheap, won't break down, and will last many moons.
Oddly enough the vehicle I depend on the most(the MIrage) is the cheapest one I've owned in probably 12 years. I saved a ton of money and time since I bought one of these. I used to normally buy cheaper junk(money pits) that needs a few things then find a ton more problems with it after the fact. If you don't want a money pit buy one of these. If people are asking $5k less than a new one then I guess that's a no brainer. Buy new and you'll get the full warranty. But likely won't actually need to use it.
View my fuel log 2014 Mirage SE wussie cvt edition. 1.2 automatic: 37.7 mpg (US) ... 16.0 km/L ... 6.2 L/100 km ... 45.3 mpg (Imp)