Warm air furnaces are tough to zone properly. The furnace has no idea it is working to heat one zone or the whole house so fan speed is too much or too little and the burner typically cycles too much. Overall, warm air heat is not the most comfortable to begin with, zoning makes it all the more less comfortable. If it were me, I'd do a baseboard hot water system. This type of system can be zoned galore, with no ill effects to comfort. Comfort actually improves with more zones. But add central air to the house and this type of HVAC can, and will, add up moneywise. It depends on what you think comfort should cost.