Somewhere between 24" tall and 30" tall, lighting seems to get potentially more expensive. Depth, more bracing area, and fewer posted relevant experiences mean more folks go with more fixtures and more power.
If you minimize headloss on the return plumbing, you can minimize your return pump, affecting initial cost and ongoing power. By the way, I like d c pumps, but I don't think you can actually get your money back on them in electricity savings
A well planned refugium and filtration system can cut ongoing costs as well. Denitrification usually is cheaper than water changes alone. I'm not sure yet, but I think so far an ats, especially if you build it and plumb it from the drain, is cheaper long term than carbon dosing for a large tank, but I think they work well in tandem with ats handling the bulk of the load. When I dose a lot of carbon and skim wet, I might as well be doing a continuous water change with my ats carrying the load, my skimmate production is comparatively light, in fact, I seem to just run it a few hours a day.
With retiree time to fill, id definitely be building my own rockwork
If you are in a cool climate, some strategically placed Styrofoam can hold in heat and limit your need for large heaters.
definitely consider if you will be around most days to feed, dose, topoff, etc or if you will be moving around. Some folks i know that retired are rarely far from home and home every night. Some like to go visit friends or family for a week or 2 a few times a year This can impact your choice of a hood and how you build your stand to limit evaporation or find a spot for a large enough ato reservoir, and maybe your sump plan
