We have the Norcold unit in our 1985 Catalina 30. It is auto-sensing dual voltage, so it can run on 110v or 12v. For our '85 we used the 'L' shape plate. We have the bi-fold teak cooler doors and they leak a lot of air. I added a strip of insulation along the hinge and also a thinner strip around the perimeter. It really helps reduce the frosting on the cold plate.
You can order things from West Marine on-line and they will ship it to your home for free if it is over (IIRC) $50.
We are still researching. The Isotherm model we are looking at doesn't have dual voltage. Something I think we want(?)
Also, tips on insulation material would be helpful. Last season I added 1" foam to the outside of the icebox and to the inside of the teak bifold lid. The icebox on our Catalina shares a wall with the outer hull so we need to insulate on the inside of the cooler. We can see condensation on the outside of the boat on a hot day, so it would definitely help.
About West Marine's free shipping, they don't ship to Mexico so we end up driving 8 hrs to Phoenix for in-store pickup/exchange.
While plugged into shore power you will be running off of your battery charger. With the engine running you will be running off of your engine alternator. I do not think you need the dual voltage item. While at anchor or under sail you will be running off of your batteries. There you will probably discover you need to upgrade your battery capacity and your shore power battery charger as well. After that you might start looking at solar panels. The fridge is just the starting place. Cold beer is worth the trouble, however.