I built a 520 about 2.5 years ago. . Will have to think about this some, though here is a start...
multiple circuits. I added a subpanel. also have a generac.
add AC to the sump room for humidity and temperature control. I live in PA, and it runs all winter.... make sure the condensation runs indoors
design everything so if it has to be replaced, it is easy to do... and keep spares of everything that is essential
and make the dimensions such that you care reach anywhere inside it
design for easy maintenance. less frequent and shorter duration are best. otherwise it will become onerous. for example I use 5 gallon buckets to hold dosing elements. that way refills are less frequent.
wait for sales to order replenishable supplies.
of course a solid RO/DI system
temperature controllers (see
https://www.jehmco.com/html/temperature_controller.html)
research all the possibilities for sumps, etc. there is more than just Rubbermaid Brute. For example:
https://www.plastic-mart.com/
investigate what is used by commercial fish farms etc. For example:
https://pentairaes.com/
Think through the plumbing and build in redundancy. use only cepex.
Also decide if you want to do water changes... as the tank gets bigger they get more expensive. There are many ways to eliminate water changes. I haven't changed water since it was set up. I use vodka and dextrose.
more as soon as they come to mind....