Apac 18Kw indoor control upgrade

Hey Guys
Looking at quoting a job using an antiquated Honeywell/Brivis T8602C1186 battery operated controller!
My client would like a more user friendly option, my thoughts would be the Leasam/Actron controller.
The old Apac is working quite well has three air zones & the Leasam controller fits the bill.
Question is how should I handle the defrost control? presently there is a four wire thermostat
24VAC, common to Stat, fan, cool, heat. they have fitted a (heat) latching relay in the outdoor to engage the comp in heating mode. However the Leasam does this inherently.
Given I have one 2.5mm2 TPS supply from the Evap fan contactor in the outdoor unit & the four wire stat what is your suggestion without running additional wiring given that I will most likely use the evap fan supply to run the Leasam PCB & hook this supply across to the fan output terminal & then use the cooling/heating section running 24V switching on the voltage free outputs.
Any advice is welcome. I did this on another Sundowner system about five years ago, though cannot remember how I achieved this!!