You have asked I think the correct questions for a good answer to follow.
You need to find out “exactly” what the NAS will be used for. If it is just SMB sharing, then practically any NAS would be fine. If you plan to build one yourself, use server grade hardware, it will give you the least amount of headaches. And if you build this, build it a month before you give it to your brother so you can run it for a month and ensure it works without issue.
Another question is: Are you going to be Technical Support for your brother? If yes, are you going to setup remote access to his machine to fix the problems or just setup changes that will be needed? And this leads to IPMI, something you would want access to if doing remote operations. IPMI is a life saver.
TrueNAS is a great NAS, if using it just as a NAS, and maybe some very light jails or applications, then a low power Supermicro board will be fine.
If you want a very simple path, buy a commercial NAS. The reason I don’t like Synology, for the money, Synology is slow. TrueNAS is fast, even on a slow piece of hardware. Using a commercial product will also ease up your responsibility to be I.T. support for your brother.
If your storage is going to be under a few TB’s, you might look into SSD’s for storage. It is silent, draws less power, and in these capacities you could create a mirror with two 2TB drives, or even two 4TB drives (depends on how much money you have). Lots of different ways to do this. Yes, you could use spinning drives, those work fine as well and are definitely less expensive.
As for RAM, of course ECC since we are shooting for Server hardware here, and I’d go for 32GB. Why? Only because your brother may see that TrueNAS can do a lot more and will want to give it a try. Why not have the RAM up front. If it will only be a NAS, 16GB RAM then would be good for several years to come.
When you install TrueNAS, use the same version you are running. It just makes life easier, and I do speak from experience. And if it’s working once you are done, do not upgrade it unless you have a valid reason to do so. Software updates can break things. TrueNAS upgrades are very good these days, but there was a time when it wasn’t. Just play it safe.
Everything I just said means a lot of money invested. This is why so many people use non-server hardware for a home NAS. With that I’d like to say this one thing… Of all these items which need to be purchased, there are the consumable parts, Fans, Hard Drives/SSD/NVMe. Wow, short list. You can expect to replace the drives likely before any fans fail, but fans will fail, including the on in the power supply, unless it is fanless. With that said, buy the good hardware that will last you 10+ years.
I don’t think I answered all your questions but hopefully I gave you enough to run with.
Do not forget the UPS !!! Critical component, and the battery is a consumable, life 3 to 5 years (typically) just sitting there not doing much.