I'm looking at houses in the Warren-Roseville-Eastpointe area and more often then not I am seeing houses that either have no central air or have 30+yo furnaces. What's the going rate for a furnace and central air install these days? Looking mainly at 900-1200sq ft ranch and bungalows.

Not an HVAC tech, but if you want the best performance it helps to run returns from each room that has a duct going to it.  Lots of old houses have a duct or two going upstairs, however there is no air circulation so it still feels muggy and humid.

My house was made in the 1850's and the previous owner had send and return ducts ran up through a corner of one house to the attic, then ducts run to each upstairs room.

I work for the largest HVAC distributor in Michigan. We sell to the contractors. Plan on $8-10,000 at least. If you have crappy duct runs, seriously consider using a multi-headed ductless unit. My house was built in 1994, but the duct runs upstairs are totally undersized. I just had a 2 head ductless system installed 2 weeks ago and it has made a world of difference. They are efficient and really can make your whole home more comfortable.