IMO, the hardware can be easily swapped as needed and upgraded where need.

For the OS its a little “easier” to use and providers a little more “flexibility” as you have more choices in what to install. There are some great videos on YouTube that I recommend to get you started. Have a look at spaceinvaders videos for pretty much everything if you go this route.

I just recently re-built my custom server and while it was the right choice for me, I always encourage my friends who are entering this hobby to think carefully about this decision.

tl;dr Get a used PC off ebay if you’re wanting to save space or money in the short term/go custom otherwise. Sample Custom Build; see the hardware section for details.

#Custom Build Pros:

• More flexible- you can pick the exact parts that make sense for you and your needs

• ‘Easy’ future upgrades- some components (think case and power supply) are a one time or decades long purchase, so future upgrades are cheaper.

#Custom Build Cons:

• Potentially higher ‘start-up’ costs - If you’re getting a brand new NAS that’s probably going to be just as expensive, or at least have a higher price/performance cost. But you can get a cheap used optiplex/thinkcentre (like you said) off ebay for significantly less than you can build a new computer. If you’re just getting into this hobby and aren’t sure if it’s for you. just buy a used optiplex/thinkcentre/elitedesk PC off ebay and roll with that for now. Make sure it has at LEAST a 7th gen Intel CPU or newer (and really, emphasis on or newer. 12th Gen or newer is preferred if it’s not too much of a premium. Stop reading here.

• “More flexible” can also mean more decisions to have to make an analysis-paralsysis.

• You’re probably not going to be able to build as compact a device, so if space is a concern, take that into account.

#Recommendations: You haven’t given tons of details on what you’re looking to do or a budget, but I think I’ve got a similar use case, so hopefully the below is helpful.

#Hardware:

• Get the biggest damn case you can afford. I bought a Norco RPC-4220 and I love it. Tons of drive slots and hot swappable/externally accessible means as my storage needs grow I can very easily pop-in a new drive without having to access the internals of the case. This specific case can be very hard to find, so unless you get lucky I would suggest you just look for similar cases. If you’d prefer a more traditional ‘tower’ style case, the Fractal Define 7XL can hold up to 18 HDs.

• Video Card- I think you’re smart to want to use QuickSync. I previously had an Nvidia GPU Geforce GPU and used the NVENC Patch to allow for unlimited transcodes, and even purchased a Quadro card later on. Now I have a 12th gen Intel CPU and use QuickSync and it’s just as good and significantly more power friendly.

• CPU- Get the ‘best’ Intel CPU that you can afford that has Intel UHD 730, 750, or 770 integrated graphics. PCPartPicker Link. I say ‘best’ because without knowing what else (if anything) you’re going to do with this computer, it’s hard to say at what level of performance you’re ‘wasting’ your money. I personally went with a 12700k because it was on sale and cheaper than the 12500 I was originally planning on getting.

• Motherboard- Get one that has the features you’re looking for at a price point you can afford, but don’t worry too much about the count of SATA ports. Assuming your motherboard has enough available PCI-E slots, you can just buy addin cards (see Faceman’s comments. Newegg and Microcenter frequently have bundle deals on CPU/Motherboards that can be a lot cheaper than buying them separate. Not all bundles are good deals though, so make sure to do the math!

• Memory- If you’re doing just the basic Arrs and Jellyfin in some kind of Linux distro, you can probably get by with as little as 8GB of RAM. But you mentioned Docker, so I assume there’s the potential adding additional services down the line and if that’s the case you’ll have to do the math on how much RAM you need. It sounds like you already have some kind of setup, so I would encourage you to monitor your current RAM usage and figure this out. I personally went with 32GB since my budget allowed for it. DDR4/DDR5 will largely be controlled by which mobo you decide on getting, but if you can afford it DDR5 prices have fallen off of a cliff lately and would be the most ‘future proof’ choice if you think you’ll be upgrading anytime in the next ~decade.

• Power Supply- Refer to the Cultist PSU Tier list to ensure you’re getting a ‘good’ PSU and use PCPartPicker to estimate how large of a power supply you’ll need. Keep in mind, PCP is estimating the power load, and that you probably don’t want to push your PSU to 100% of it’s power limit. I usually target power supply that can do 20% higher than the estimated load in PCP.

• Storage- Try to find CMR drives- SMR drives are cheaper but frequent writes can cause them to suffer from performance issues or premature death.

#Software:

• Go with the OS that you feel comfortable with and can effectively support. Don’t feel pressured to use an OS you’re unfamiliar with because some sweaty internet nerd told you it was ‘the best’. Having said that, this sweaty internet nerd recommends either Unraid (simple, easy to use and add storage to, supports docker) or OpenMediaVault which isn’t as ‘slick’ as Unraid, but still very beginner friendly and offers a nice graphical web interface to manage things and also has easy plugin based support for Docker (among a ton of other things). I personally went for OMV at first, and once I got more comfortable with Linux and administering my server, switched to Proxmox (Host Hypervisor) and Ubuntu Server (Guest VM).___