I run a Plex server and have for a few years now and I will tell you this. You do NOT need super fantastic hardware to get a Plex server running. I run my Plex Server on my Intel Skull trail NUC . The particular specs of my unit are as follows
CPU: i7-6670HQ (4C 8T @3.5Ghz Max)
RAM: 16GB (Ram is actually a very small factor for Plex)
As for the storage, I actually have all my content stored on my NAS. My Plex server pulls all the info it needs from my NAS, transcodes and pushes it out to wherever it has to go. This is all done over your standard 1G network. My NAS does have 10G but obviously the NIC in the NUC will be limited to 1G (Plus it makes absolutely no difference if you are sending info outward over your broadband that is not even capable of attaining those speeds which is most peoples bottleneck). What I can say is the information is pulled from my NAS that is in a RAID 0 Configuration with 7200RPM Seagate IronWolf Drives which is then further cached using 2TB SSD's for all those frequently used files. I cannot comment on how much benefit you get from the SSD caching in terms of Plex overall performance but I would hazard I guess not an awful lot unless you have loads of people watching the same content frequently. What I will say is fast storage is a must whether it be RAID 0 HDD's or the use of SSD's. If you are going to serve multiple streams concurrently, then fast storage is a big must IMHO. For latency it makes sense for your solution to have internal storage. I only serve through my NAS as I prefer a singular data source and a SFF solution.
You will notice my solution also has ZERO dedicated GPU's for transcoding assistance, all of it is done on the CPU and I am yet to have problems. Though that is not to say I do not recommend a GPU. If you can get a compatible GPU for transcoding then for sure get one as GPU's are obviously much better for that type of workload.
Most of my streams go over at 1080P but last time I tested a 4K Stream I did not see any issue in the stream when watching content and buffering was not something I ran into.
So I guess in answer to your questions, CPU's are still very relevant and capable of providing Plex transcodes, GPU's will clearly benefit the trascoding overall so there is no reason not to get one. If it were me I would get a a capable CPU in any case should you have to fall back on to it if the GPU is having issues or being swapped out. Remember that the CPU will has to be good enough not too hold the GPU back as the CPU still has to babysit the overall process, so yeah don't skimp on that CPU in favor of GPU.
As for 4K/60 Transcoding I would recommend Pascal minimum for future proofing.
Hopefully this helps a little, can only go on my experience