Build your own walk in cooler.

AC units are more powerful than most fridge/freezer units, as they are designed to cool a much larger, less insulated space, but they are not tuned for the type of work required for a freezer. They will be terribly inefficient unless you know enough about those systems to pick exactly the right size one and tune it properly.

When buying a walk-in cooler, you’re also buying all the insulation and the properly sealing door, which is as essential to the operation of a fridge/freezer as the heat pump. Those need to be planned out even more carefully (although overbuilding will pay off in the end, unlike getting a too-powerful heat pump). A really well insulated, properly sealed box could run off ice instead of a heat pump.

On the saving electricity claim, a new AC unit might be more efficient than an old freezer unit, but if comparing two new units, the efficiency ratings should be very easy to find out and do a straight comparison on. But as well as looking at the power efficiency of how many watts of power do you use to move one watt of heat, also you want the power cycle to obtain the desired temp to fall well within the manufacture’s expected range to keep that unit working a long time without service.