A walk-in cooler in a 10 feets basement : Temperature and Coolbot

Hi everyone,
We are just starting to sell veggies so we are in need of a walk-in cooler. We already have a huge room (12' by 18'), in our basement that is 10 feets deep on rought cement. We are planning to insulate the ceiling and the first 4 feets of the wall this week so it would already be quite cold (it is already), but we don't know how much. If it's not gonna be under the 10°C/50°F, we would need a Air conditionner with a coolbot (or an other cooling system that we haven't yet discovered). We also wonder if, because of the huge size of the room, we would need an extra power full mega btu machine or because the temperature is already low a normal one would do the job. Also, the cement room dosen't have a window, but it as some holes, so we are looking for a mini-split alike systeme (or an impressive portable air conditionner).

Thanks a lot for your advices!