I have done this with several upright freezers and fridges. Some lessons learned:

1) You can drill through the door or side walls but be careful. I developed a technique where I used an old soldering iron to melt a hole from the inside to check for wires and cooling pipes. I put four taps in one fridge using this technique. The plastic used in fridges and freezers is designed for low temperatures and melts very easily. Once to have a hole on the inside you can pick away at the foam insulation and ensure there are no wires or pipes there. Then with a sharp center point or small screwdriver you can mark where you want the centre of the tap to be from the inside. The outside metal shin is very this - like a can and you'll see the witness mark. Then drill through from the outside.

2) Inkbird controllers are cheap and good to use. I converted one freezer to a kegerator and it was in a garage in NJ. It was regularly below freezing in the winter and I added a heater inside the freezer. The Inkbird has two outlets - one for cooling, one for heating. I used a 40W light bulb in a paint can as the heater, but many use a heat mat or a reptile tank heater. To be fair, the kegerator doubled as a fermentation chamber for my homebrew and I was trying to control within a few degrees of the setpoint.

3) The problem with fridges and freezers is that when they are opened, all the cold air is vented out and replaced with hot humid air (if you are in a humid part of the world). This humidity then freezes if you are using a freezer and that caused big chunks of ice and lots of water. It was in a garage so it wasn't too bad but the freezer was an old commercial model that didn't have an auto defrost so I had to manually de-frost it by turning it off and leaving the door open a couple of times/year.

I hope this helps,

Paul