most shouldnt be put in cabinets and i wouldnt worry about 81. you would be fine getting to 84-85 you just dont want it falling to 78 from there. 84 during the day and 80-81 at night would be fine any lower you would want to monitor coral for reactions to the drop and maybe up the heater to control the low point.
and as scsi said chiller and inexpensive dont appear together