A.,
You could get some thick tights for the bottoms, even for under jeans. When it gets snowy, have some snow pants & boots for her, or if boots are unrealistic, you can put her feet into baggies then put on her shoes. It'd keep her feet dry, sometimes boots are too hard for new walkers to wear. I'd say stock up on some cute tights though. The cable-knit type are very warm.
Have fun!
D.
PS for the boots in the snowsuit question, I'd day no, put their socks on pulled up then snowsuit & boots. If you need to readjust them you can pull the snowsuit out of the boots but you'd really have a hard time getting the boots through the legs. And ... welcome to Chicagoland! Find a good hill & get some lightweight sleds, you'll have a blast when the snow comes, just don't forget a thermos of cocoa! :)