Hottest Day of the year in MI and I'm thinking about winter

I know there's threads here already, but I still have some questions. I've never used winter tires before, so could use some advice. It seems like going down to 18" makes sense. I guess I'll get a set of cheap rims for $120 each. I think I saw TPMs on the chevy site. I guess I have to buy a set of those too? A couple of the tire websites are telling me the back tires and front are different sizes ?!? Mine aren't now on the 20s. Can't the 18s be all the same? Before I go get some dude's advice at a tire shop, I just want to know what's best (cheapest + safest).