What hardware do i need

Hi all,
I am fairly new to plex and could do with some advice on how best to setup my system for future use.
I currently run plex server on my iMac (2.8ghz i5) streaming movies etc primarily to my samsung smart tv. I wanted to up my external hard drive capacity somewhat, to cope with some 4k content that may come along. What is the best solution for me, do i want to fork out for a smaller but faster ssd or am i better off with a higher capacity non solid state drive? I assume i do not not need an NAS drive as the iMAC should handle any transcoding or whatever, or maybe a new NAS would surpass my iMac in terms of processing power for this task?
Im sure its clear but I’m fairly new to this whole media server setup, but i can see a lot of plus points in using plex, so just trying to get myself up to speed. So any advice is welcome, no matter how basic.

Thanks all

Conor

So if I read correctly you’re simply looking to increase storage capacity?
If so then there is no need for SSD.
The read speeds on modern mechanical drives will easily cope with high bitrate 4K media.
You don’t say what the CPU is in the iMac.
If it’s anything KabyLake onwards you should have no issues at all.
But if you only plan to direct play it probably isn’t that important anyway.

A NAS doesn’t do any transcoding (usually, most NAS devices don’t have suitably powerful CPUs to do it).
But it still can serve you as a giant media storage container.
You just point your Plex library on the Mac to the NAS as a media source.