NAS: self-collection or ready-made (details in the description)?

Question

NAS

Hello.
I want to organize network storage. The needs are standard: in fact, data storage (including backups), distribution of torrents (about 50 pieces, including voluminous series), distribution of content to a TV and smartphone. Number of compartments: 4, because I want to insert existing disks from the computer's system unit there (the volume is different).
RAID is not going to organize. It does not cancel the creation of backups, so I do not want to waste space. I just want to connect data disks and use it.
I already got acquainted with NAS4Free on a virtual machine and even played around with ZFS. I’m ready to spend time setting up the environment I need in combat conditions.
What I focus on from the finished one: Asustor AS-1004T or similar. This model in my city costs 16,300 everwood. Let the more expensive models go through the woods - for their cost you can arrange a trip, which is clearly better.
Samosbor has two scenarios.
First. The hardware of the main computer is updated, the existing NAS departs. But for NAS, this is the most available is clearly too much: i5 750 and 8 GB DDR3. I'm not going to spin virtual machines on NAS. Theoretically, of course, they can appear, because in general all sorts of such things are interesting to me, but now I just want a repository.
Second. The ASUS N3150I-C motherboard is purchased (4700 rubles), possibly a case and a suitable power supply. RAM, just the DDR3 generation, is already available from another computer in the amount of 4 GB. The case is in the "possible" status because the hardware of the main computer is planned to be upgraded anyway. But even if we take, for example, Fractal Design Node 304 (5100 rubles), then without taking into account the power supply, this option will cost me only 9800 rubles.
The issue with the power supply is relevant for both the first and second options, and for me it is the biggest snag. I read a lot of materials on the topic (mostly this is the iXBT profile conference and articles and comments on them on Habré), and I myself understand that in such a configuration the consumption is scanty. Install an ATX PSU? Seems like a lot. Even at 350-400 watts. External PSU? Then you need a motherboard with a DC input. It is possible (for example, AsRock Q1900DC), but there, in the models I saw, the SO-DIMM RAM size is used and I don’t want to spend money on it at all, especially since it already exists. Even in all "simple" ITX motherboards, the number of SATA ports is not satisfactory. And servers are expensive.
Actually, questions.
Tell me, please, how to be with the power supply?
If you take an mATX motherboard for the second option (with an LGA socket, for example MSI H81M-P33) and put an Intel Celeron G1840 there, isn't the heat pack for NAS too fat? In terms of money, this is 9550 rubles (with and without a PSU).
If you still take a ready-made NAS, is there enough power for the voiced tasks? I tried to find this information on the Internet, but reviewers indicate only the transfer speed over the network. It seems to me that 512 MB of RAM will not be enough (or rather, very little).
Well, just give your opinion.
PS About HP Microserver in the know. Didn't look very closely. For now it stops that compatible RAM is needed. The server itself in my area costs around 18000.
If you're wondering (or even important) what the main computer upgrade plans are, they include buying a smaller case (because there's only an SSD for the system and one hard drive for hot data and other toys; the rest will move to NAS) for ITX- board with socket and DDR4. Accordingly, the case, as I said above, can be given under the NAS.
You can also rightly note that I did not mention the secondary market, where you can find cheaper NAS. It's right. Do you think it's worth a look there?