Merkur wheels have an offset in the mid 30's as I recall...about 15-20mm off from an OE Focus. They may have some rubbage issues because of that. The Focus is happy with offsets in the mid 40s, maybe down to 40--after that may be an issue.
Haha Wow, I thought I was the only one who wanted to put Merkur wheels on my focus. My dad's got a Merkur xr-4ti sitting in his yard waiting to be rebuilt, and I have been eyeing them everytime I go to their place. They are 18's though so I am pretty sure they would be too tall, I was also concerned about them rubbing just because of the style.
They would have fit my old VW Rabbit though. hehe =]