With that fridge you're just adding to the complexity of your set-up. You want to add a fridge, but in order to do so, you've got to first add a fuel tank. Once you've installed the fuel tank, you've got to install fuel lines. Once you're on the trail, you've got to worry about fuel consumption and filling.
With something like the Engel/ARB, you don't have to worry about that. If your deep cycle battery starts to get low, just charge it with your vehicle. Plus the Engel/ARB has only one moving part, so if the fridge should quit working for some reason you know what the problem is.
I'll agree with you that these fridge/freezers are a lot of money. But this is not an area to cut costs. Don't get cheap on the fridge. If you can't afford it, toss your igloo cooler in there for now. Don't waste your money on that fridge. Don't be a cheap****.