If you are looking for input, here's what I feel should happen to give the feel of what happens in real life:
1. Units have a 'range' based on turns to live (or their initial provisions)
2. Units will suffer decreased movement and lower combat strength as they use up their supplies.
3. Lines of Supply can only be setup where there are no enemy units, since they are a choice military target.
4. Supply units can bring a unit back up to strength quickly
5. Supply depots can be setup in remote locations
Now to piece this together for usability in the game...
See next post for details.