I'll be moving to Iowa City in August, but I've lived in Des Moines the past six years and I assume the weather is about the same. Here's my advice: Cold is cold; how heavily you dress will depend on how tolerant you are to it. The real problem is the wind. There are winter days when the temperature alone would be quite comfortable without a coat, but the wind will numb ever sliver of flesh you leave exposed. Definitely consider how well your winter gear will block that wind before purchasing.
Also, I'm hoping that Iowa City cleans up the snow a little better than Des Moines does. I walk pretty much everywhere I go, and in Des Moines it hasn't been unusual to be treading on several inches of ice for 3-4 months straight--one year it was from November to April. If you aren't used to it, you should be very careful when walking (or driving!) in the winter.
