+1 on the RO/DI and the LED lighting. Electricity costs are skyrocketing and MH/T5 bulb changes a few times a year can add up to a few hundred dollars annually; this is what makes the high up-front costs of LED worth it.
I built my system to be as automated and simple as possible (with a lot of overkill). I am just shy of $100.00 per DT gallon in my setup but everything I purchased will easily transfer to a much larger tank down the road. I went with a DIY turf scrubber instead of a protein skimmer (saved a couple hundred dollars), but I still need level switches for an ATO as well as some pumps for AWC and haven't even started to look at a calcium reactor or two part dosing.
All the little things needed to get started will put quite a dent into your wallet as well. Refractometer, test kits, nets, gloves, tweezers, tongs, cleaning magnets, measuring spoons/cups, buckets/pails/small containers, towels/rags, feeding utensils, syringes, basters, etc. Don't forget an extra heater and pump in case of equipment failures and to heat and mix your new SW batches.
You really need a QT as well, I managed to pick up a used 30g with a HOB filter, heater, thermometer and stand for $75. None of the QT gear can be used in your main system to prevent contamination. Once you are at that point you can start looking at livestock along with all the types of food available.
I researched everything I could for 7 years, knew every single item I wanted and it's cost before making a purchase and still managed to run about $2500 over budget. I did end up with a used Profilux 2 though, not necessary to start out with a controller, but I can say it has been worth every penny as a newb to the hobby.
Best of luck.
PS - I approached my wife the right way with this hobby and got total buy-in. She purchased an MP40 for me as my first item, now if I can just get her impulse buying behavior for livestock under control........
