Generally power supplies are the most electrically efficient at 20-60% utilization, so there’s no issue with over-provisioning power, other than the (generally minor) upfront extra cost, which might very well pay for itself in the first months/years of usage. I’ll take a look and see what I can find on those sites.

Edit: okay, trying to shop through google translate / currency calculator is actually aids so I’m gonna teach a man to fish instead. This is what I should have done from the start anyway.

Power supply: Anything from a decent brand, at basically anything >450W. a 650W or 850W is totally fine if it’s at a decent price. They only draw the power they need, they don’t just constantly pull 850W if the downstream components aren’t calling for it.

CPU: 12400 is a fine cpu for what you’re doing. You’ll transcode at 720p no problem, 1080p maybe a single stream in real-time. I wouldn’t bank on more than that. Only downsides here are the relatively shallow core counts if you ever expanded into other workloads. Without access to used xeon boards/cpus, it might be a reasonable choice though. What I would say is look for something older but with more cores/threads if you can. For example, a 10900 or even 10700k would probably be a better server cpu than a 12400.

Memory: DDR4 platforms are a great way to save money, as long as you aren’t planning on expanding to inferencing on cpu. Get as much as you can. 32-64gb of ddr4 should be dirt cheap, especially if you find a cheap motherboard with 4 memory sockets.

Motherboard: If you want this thing to be versatile, you want 2x pci-e slots. Old gaming full-sized ATX boards are the way to go here. 1 slot for an HBA, 1 slot for a GPU, and that should be all you need. Bonus for as many open sata sockets as possible. 6-8 is pretty typical on 10th-12th gen gaming ATX boards.

GPU: gpus will be much more efficient at transcoding than an igpu, especially from older intel CPUs. A 1050, 2060, 3050, basically anything from the 10-series onward has a decent nvenc encoder that would work well with plex/jellyfin. My goto is generally old workstation cards, I use a p620 myself and it handles a single 4k encode job no problem. I’m not sure if they’re viably purchasable anywhere in your area, but I’d definitely look out for a P620, P1000, or T400. Great value in those cards.

Drives/HBA: there are inexpensive LSI HBA cards to expand how many drives you can attach to a system if you need them, all you need is a spare pci-e slot and a place to physically mount the drives. The cheapest way to start here is to look for a motherboard with 4-6 sata slots and use those. Hardware raid is functionally dead these days in the real world, just use zfs or mdadm under linux to create an array with your desired level of resiliency/capacity.

Once you’ve priced out what it would cost to buy all of this new, look for prebuilt gaming PCs and office PCs that might be able to be expanded to fit these requirements. Prices look kind of steep on those markets you listed, but I’m sure something exists if you look hard enough.