Yeah.. replace the blower motor first. My Explorer actually smoked out my interior on setting 2, and after that it only worked on the highest setting or nothing lol. It's common for blowers to do that.
Most houses I have dealt with north of the Mason-Dixon line have one air handler for the whole place, even a 3 bedroom townhouse with a full finished basement. The return is high up on the wall where the wrap-around staircase is located.