I need 24 tb usable space after raid.
If you need 24 TB usable space, you could go with 6x 8TB or 7x 6TB HDDs in RAIDZ2.
RAIDZ2 uses 2 disks worth of space for parity.
And you should not fill your pool more than about 80%, plus the file system needs some space too.
The 6x 8TB configuration has 32TB net capacity, 80% would be 25.6TB.
The 7x 6TB configuration has 30TB net capacity, 80% would be 24TB.
Of course you could also use 5x 8TB, 6x 6TB or 8x 4TB, all of them give 24TB of capacity, but you shouldn't fill them up completely.
As you want to use Plex, do you need UHD or HD transcoding, or can you play your media as-is?
If you need transcoding, then you would need a huge load of CPU power, especially for UHD content.
Or you could wait until IGP-assisted transcoding is available in Plex/FreeNAS, I read this is work in progress, but I don't know when it will finally be available.
GPU-assisted transcoding will most likely never ever happen, so a graphics card is no use.
Do you provide some used configuration server spec?
Regarding used parts, it depends on your local market.
I'm in a similar situation right now, and in the process of deciding between used and new, so here is a part of what I came up with (all Micro-ATX boards):
- If I were buying new today, I would choose a
- Supermicro X11SSH-F mainboard (C236 chipset, socket 1151, 4x DDR4 UDIMM, 8x SATA, 1x M.2 PCI-E (for a small boot SSD))
- Intel Pentium Gold G4560 (2x 3.5GHz), Core i3-7100 (2x 3.9GHz) or Skylake (v5) / Kaby Lake (v6) Xeon E3 (maybe upgrade later if you really need the transcoding feature and still lack power)
- Samsung DIMM 16GB, DDR4-2400, CL17-17-17, ECC (M391A2K43BB1-CRC)
- Or wait a little until the new mainboard generation becomes more widely available (you said you wanted to buy step by step)
- Supermicro X11SCH-F (C246 chipset, socket 1151v2, 4x DDR4 UDIMM, 8x SATA, 2x M.2 PCI-E)
- Intel Pentium Gold G5400 (2x 3.7GHz), Core i3-8100 (4x 3.6GHz) or Coffee LakeXeon E3 (maybe upgrade later if you really need the transcoding feature and still lack power)
- Samsung DIMM 16GB, DDR4-2400, CL17-17-17, ECC (M391A2K43BB1-CRC)
- Used Supermicro X9 boards are often recommended here, but are rarely on ebay in Germany (where I live), and are about half the retail price of the up-to-date models.
- Supermicro X9SCL(+)-F (C202 chipset, socket 1155, 4x DDR3 UDIMM, 6x SATA) or X9SCM(+)-F (C204 chipset, socket 1155, 4x DDR3 UDIMM, 6x SATA)
- Intel Xeon E3-1225 v2 (4x 3.2GHz) or other Sandy Bridge / Ivy Bridge (v2) Xeon E3, are quite cheap on ebay in Germany
- Samsung DIMM 8GB, DDR3-1333, CL9-9-9, ECC (M391B1G73BH0-CH9) for example is on the mainboard compatibility list, but used ones are a little bit rare, and prices are almost the same as DDR4
- The X10 boards are also often recommended and newer than the X9, especially the X10SLL-F are quite often on ebay in Germany at about 1/4 the retail price of of the up-to-date models.
- Supermicro X10SLL-F (C222 chipset, socket 1150, 4x DDR3 UDIMM, 6x SATA), X10SLM-F (C224 chipset, socket 1150, 4x DDR3 UDIMM, 6x SATA) or X10SLH-F (C226 chipset, socket 1150, 4x DDR3 UDIMM, 6x SATA)
- Intel Xeon E3-1225 v3 (4x 3.2GHz) or other Haswell (v3) / Broadwell (v4) Xeon E3
- Samsung DIMM 8GB, DDR3-1600, CL11-11-11, ECC (M391B1G73BH0-CK000) also a little bit rare on ebay in Germany and not really much cheaper than DDR4
So you'll have to do a little search, what is available and how the prices are.
Sometimes there are complete kits (mainboard, CPU, RAM) on ebay which are even cheaper than if you would buy each item separate on ebay, or used servers including chassis.
Maybe
@Chris Moore can suggest some configurations and parts for you, he is the expert in picking parts.
By the way: I just think it would somehow be nice, if there were a sticky thread/document with some example configurations.
I know the hardware recommendations guide, but a more detailed list would come in handy, because questions for that come quite often.