I'm not sure if this works for anything
outside your local network, but at least inside your local network, Plex will direct stream to any device that supports it. It only transcodes as a backup, in case the target device doesn't natively support whatever codecs the file you're streaming are. It also transcodes if you have to burn in subtitles, sometimes soft subtitles don't work properly for some reason so they have to be burned in.
That said, making sure your content is natively compatible with all your devices typically can't happen. I try to transcode all my stuff to HEVC because of the smaller file size, and only the Roku supports it. I don't really stream to anything else, but you've got a lot of devices to cover, so transcoding is probably gonna happen anyway. The good news Plex supports NVENC and Quicksync (not sure if AMD transcoding is supported, doesn't seem to be).
Not sure if you have any hardware laying around, but if you're buying all new, I suggest the i3 8100, 2x4GB of the cheapest DDR4 you can find, any H370 board, or even H310 board if you're okay with software RAID... this
power supply is pretty good... The graphics chip on that i3 8100 is the newest available from Intel with the latest version of Quicksync. It even works for transcoding HEVC, but I'm not sure how many streams it can handle simultaneously...
Most onboard RAID supports RAID 0, 1, 5, and 10. Raid 0 is terrible for this case, if one drive goes, you lose everything. Raid 1 is a mirror, so if you RAID 1 two 4TB drives, you still have 4TB. RAID 1 4 4TB drives, you get 8TB. RAID 10 is a RAID 1 of RAID 0 arrays... not really useful unless you need serious
write speed.
Read speed will still go up with RAID 1. RAID 5 will deduct one drive from your total storage space for parity and thus allow you to sustain one drive failure, but it's not recommended anymore with the size of today's hard drives. Hard drives are getting so large these days you run the risk of running into a URE (Unrecoverable Read Error) in the event one of your drives does die and you replace it and rebuild the array... if that happens, your data is toast. RAID 6 solves that problem for now, until hard drives get too big for even RAID 6, but it's not really available outside of hardware RAID cards, which are expensive. In short, your best bet would be to stick with RAID 1. However much storage you need, buy two of that drive.