Yes you need one. And the world Router is self explanatory.

Get a Synology NAS. A 4 bay if you have money, avoid the one with ARM CPU, get one with an Intel CPU.

You didn’t say where you live and what budget you have. What solution do you want to plan, what HW etc. So I assume you live in Europe, amazon.de, going shuking, you would need 10x18TB drives for 292€ each, total 2920€. Then you can arrange it on RAID 5 and get 162TB, RAID 6 and get 144TB, or unRaid with double parity and get 144TB.

I just Google on amazon.de and do some basic math. It didn’t take too much.

You are confusing yourself with hardware and different standards. SATA 3 can max out at 6Gb/s. Generic HDD works around 220mb/s, 1G network is 110mb/s, 2.5G network is around 300mb/s.

So to give you a perspective, if you have 1G as a local network, you can’t max out a HDD capability, with 2.5G you double the performance, saturating the HDD, if you want more you need a generic 2.5" SSD, if you want to saturate the SSD you need 10G, in that case M2 SSD start to make sense.

Remember that if you want to go 2.5G, you need all the infrastructure of your home to work around that speed, not only the Nas.

The DS1821 runs with Ryzen, so no transcoding capability at all. Anyway, most Intel solutions sell on those prebuilt NAS, don’t have good performance, mostly enough for the basic Nas function, and mostly old stuff, without all codec support. You need an Intel CPU, one with integrated iGPU, capable of Quickl Sink.

If you need transcoding power, better go DIY. To give you a perspective, an i5 8400 can transcode around 4/5 4K video at the same time. As 1080p, we are over 20+ at the same time. That’s the UHD730 power.

No. If you don’t run strange software that someone give you or open link and download stuff, no.

Your is just a WiFi problem.