It helps if you FIRST have a chat with the local health department. It matters just what the cooler will be used for.
The first decision you must make is: Inside our outside? It's best if you can run your wire outside the cooler, but that's not always possible. There's less opportunity for crud to accumulate (health issue), or someone to damage it with a hand truck.
Where you DO penetrate the wall, you will probably want to use duct seal to seal the pipes where they pas through the wall - prevents moisture from condensing into the pipes.
Once inside, clean-up us a major concern. While a 'beer cooler,' as seen at the local 7-11, may do just fine with pipe run directly on the walls and ceiling .... if there's unpackaged food handled in there, you'll probably be required to run the pipe suspended away from the walls and ceilings. Mineralacs might satisfy the health department ... or you might need to go into the local plumbing house for those larger plastic stand-offs. They need to be able to clean behind the pipes.
Boxes need to be "bell" type, again for sanitary reasons. You want as smooth an exposed face as possible. Fixtures need to be totally enclosed.
Each evaporator fan needs a disconnect switch. In freezers, there also need to be receptacles for the heat tapes on the door and drain lines.
I would not consider using anything but EMT, with sealtight where making the transition to the evaporators.
I have never even heard of a problem where the boxes are mounted to the wall - the stuff has to contact the walls SOMEwhere. You can't eliminate EVERY place that might accumulate crud ... but you can reduce them.
Any wiring method has to be suitable for the mechanical and other environmental hazards it will encounter - back to Article 110 - so PVC in a freezer might be a violation. I don't like the idea of PVC ... but I'm from Chicago
And ... if you haven't already guessed ... GFCI's and bubble covers on the receptacles are in order.
