Just to add to Magoo's input,
It is recommended to have lots of evaporator surface area & capacity, especially with a product like ice cream where temps are critical.
If people go for cheap option you will never get room temps, due to defrosting all the time in your climate.
Penthouse unit evaporators are also a good investment so you can work on them with ease.
Rapid roller doors on freezers also worth investing in.
Ammonia would be the best but not cheapest on initial installation.
Ammonia would be the most efficient to run with as well as refrigerant price.
Electricity cost is also a major part of the design to keep running cost to a minimum.

There is a lot to say but hopefully your consultant knows his stuff.
All other refrigerants very expensive.
You would also want built in redundancy if something goes wrong, or for maintenance.

eg more than 1 condenser & compressor, so you have back up.