transcoding from what I know is cpu intensive so go all out in that area if I was you , also if you have a plex pass you are able to set it to hardware acceleration just so if your budget is tight and cant go all out on the cpu maybe a half decent cpu and gpu combined to do the work , also another tip from what I think is correct , you can change the location folder of where plex transcodes its files and if you can get your hands on a m.2 ssd and your board allows this (you can get pcie m.2 expansion cards) I do believe setting your folder to the m.2 will also help keep the speeds up as what I do know is transcoding 4k films can be intensive for the machine .I'm hoping to get a good deal on because I'm in a tight budget. No more than USD $250.
Right now I have a PC with AMD A8-7670k CPU, MSI A88X-G45 motherboard and 8GB of ram.
Windows 10 is running on a Samsung 250 GB SSD. All data is saved on a LenovoEMC NAS with 12GB. PSU is a Thermal Take with 430watts. TR2-430NL1NH
I do not play games so the most difficult task would be to transcode and decode videos in 4K to 720p for remote watching using PLEX or any other media server app available.
I have an old case that came with an old Compaq PC that I've been using to build my PCs because is made of pure aluminum.