1. More stages is better. If you are trying to cut costs to stay under 500k JPY I run a 5 Stage in Florida with no Issue and I could run a 4-stage as many at my LFS use that at home with no issue. The price difference is not that crazy but with lights it may increase above 500k so that's why I mentioned it. You really only buy it once so the 6 stage may be worth it out of caution. Do you have a TDS meter to measure once the water is truly RODI? Do you have much experience with RODI units. Usually it takes a bit of time before when running water through it for it to hit low TDS numbers for the water to be really RODI.
2. I may have missed it but where is the AIO back in the list of items. Sorry I am not sure if its in one of the items in Japanese and I may have missed it lol.
3. That should be fine I would take it slow with refug as you do not want your nitrates to bottom out as well. Macroalgae can be added after you have the system running with fish but a live cycled rock is usually better than bio pellets or even a dry rock you add while cycling would be better than the biopellets (you can take a dry rock and smash it so it fits as well.
4. If its common and cheap then that works. Instant ocean is like half the price here and would work the same. It may not be avaiable in Japan:
Here is Link on Amazon
5. That makes sense. Just be careful not to overdose as with that Redsea salt the levels will already be high for calcium, Alk, & magnesium.
6a. That is strange. I know in other countries many do not have Air conditioning but I would just make sure there is good gas exchange (openings above the tank/lid) but if the room temperature is above 76 than you could definitely run into heating issues. I do not have much experience with cooling systems but I have been told that they do fail so just be aware of that.
6b. Redsea 50s would work, I trust both brands for lights. AI gives you a lot more customization but both would work.
7. All for reef is good just make sure to have some other product to increase PH and make sure to monitor it. People usually use kalkwasser or CO2 Scrubber to keep the PH up. Essentially every time you dose All for Reef it will slightly decrease the PH so unless you are doing regular water changes it can drop low.
Here is an example Link of what I mean. Also there are a lot of forum posts on it. Usually it is recommended for 20-30 gallon tanks but there are Redsea Foundation products that are for each element that would not run into that issue. (Although that may impact the dosing machine options)