:hi:
the focus bolt pattern is very unique. it is basically model specific and they never offered the focus with 16" steel wheels. The only sizes that you can get at 14" and 15". If you are looking to use snow tires, this is what it comes down to: Narrower is better.
A narrower snow tire will "cut" through the snow better then a wider tire. With a 16" wheel you will be looking at a 205 width tire while the 15" will have a 195 width tire. And if you go down to the 14" tire its 185 width.
The next thing is cost:
Rim wise you have a few options in order of highest cost to lowest:
(***these numbers may be a bit off, but provide and idea of where you are at***)
1) find a set of 16" aluminum rims--used or new you are looking at anywhere from $350-600+, places to look for used would be ebay or at you local wreckers
2)find a set of 15" aluminum rims--$350-$600 look for same as above
3) find a set of 15" steelies (im pretty sure they are available) New $70 apiece and last time i checked the dealer and Canadian Tire had the same price for them. You may also be able to find them at a wreckers for cheaper
4) find a set of 14" steelies--new they are $50-60 apiece or you can check a wrecker. also there is a member selling them
HERE for a really good price (***i know the ST runs 4 wheel disc brakes and I dont know if it can accomodate 14" wheels***)
The next cost is tires: This is where it ranges the most.
Its gonna matter alot on what brand of tire and which model you choose. The higher the model you choose in their range, the more expensive the tire regardless of size. That being said, I dont remember seeing too many "cheap" 16" snow tires last time i checked regardless of brand, and some brands dont make them in that size or they may be a special order depending on where you go and what brand you choose. The 15" tires will give you a little more choice in the tire and how much they cost. Same goes for the 14"'s if you can run them.
A lot of places, including the dealer, usually offer winter rim and tire packages that usually shave off some of the cost of going all brand new so check around those places.
If you looking for some good winter tires, I'll let the other guys chime in in the morning with their recomendations in the morning.
personally i would go with the 15"s because they cut through the snow better and dont "float" as much as the 16's with the 205 width tire. They will probably end up being a little cheaper also.
I think that pretty much covers all of your options and if you have any more Questions, post em up!!
Enjoy your stay!