Avoid mini PCs if you want upgrades later on IMO.

The SFF or MT form factors are a lot better, I’d say MT is the best as it has full height PCIe slots. Keep in mind the dell/hp/lenovo models all use proprietary motherboard form factors and power supplies, but not that big of a deal I think since there are so many parts available if something does break.

I highly recommend 7th gen intel or newer, as you get the much better quicksync support and quality.

If you get a desktop class CPU (i5-7500 for example), the whole computer will typically draw around 15W at idle with an SSD, which is pretty decent.

If you need less idle power draw then you’ll want a mobile/notebook class CPU (like an i3-7100u) as the idle usage should be less than 5W. But those typically only come in the micro/mini form factors.

Also good to remember that every 3.5" HDD draws around 7W when idle and spun-up (typically difficult to spin down on servers since there’s always some process accessing files).