help diagnosing blower motor please...

I have a 1981 300TD Turbodiesel wagon.
When I first crank the engine over, the blower goes briefly but is rattling. When I change any settings, it quits and will not come back on. I feel that the push buttons are working as I can hear the vents opening and closing when I push them. Also, there is heat that leaks out of the vents after the car warms up but, again, no blower to really push it out. Nothing will get the blower working, not even the defrost function.
When I go to Fast Lane it appears that I can replace the motor and the fan separately. How can I diagnose further? Is there a possibility that I can get away with repairing or replacing something other than the whole thing?