IMHO it will make it easier to sell if the current unit is a 30 year old strange looking monster of a unit. As for dollar for dollar return I doubt it - but it was easier for me to buy my home since the HVAC was a quality newer setup. just $.02
With most improvements there is a certian ROI - both in dollars and sellability. Here you are probably looking at more adding to the sellability if an older unit being replaced with new highly efficient unit. Unless the house does not now have central AC and you are adding that - then there is a bigger dollar ROI.