Building a UPS with Deep Cycle Marine Batteries

I'm looking to build a UPS from a few deep cycle batteries. The ones I had in mind are the Deka Gel Dominators since they are a local company
that is literally on the same road I travel on my way too and from work. As much as I would love to get a commercial unit we're on a budget at the moment. Each machine will use about 300 watts and we're looking to start off with two machines at the moment. What will I need to do this properly? I saw one on Youtube for a HAM radio operator that had a LCD screen showing how much time he had left at the current level of draw. What would I need to do this properly?
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