15's will (almost certainly) not fit. Supposedly there are 15's with thin enough rim material to clear the caliper, but not many.
Used 16" stockers are what most poeple use, I got a set off ebay for $230 shipped, and looking at how tight it is (I cant fit my pinky finger between the caliper and the inside of the rim on the front) I can see why 15's wouldnt fit. Too bad, cause the tires would be way cheaper.
Hmm, research is a little hard to dig up, partly because very few owners have tried anything besides 16" alloys.
I dont think the steel wheels would work either- look at our spare. It would've been way cheaper to make it a stamped steel wheel, but its some wierd custom 16" alloy thing. Our calipers are a pain in the balls, for whatever reason they're much deeper than the regular focus caliper. TireRack wont sell a steel wheel in any size for the SVT, because most of them will hit the caliper.
Doesnt mean they ALL wont work, but most wont and nobody wants to be the guy who spends a few hundred bucks on wheels that end up useless.
Yep, like the others said, get the cheapest 16" you can find in the right offset. The links WD put up talk about the offset a little more. I run 16" stockers I painted black in the winter with 205/55's and they've worked awesome the last two winters. Watch ebay for awhile to get a good deal, especially if you can find something local to avoid shipping. And watch out for the guys selling just one wheel for like $180 on ebay, that's a rip. I got mine for $175 from a guy on ebay that was local.