SSD Cache Pcle v Sata?

[Enquiries]

I've just bought a QNAP TS 673 and want to set up a dedicated Plex server with 2 of the drives. - It has 4 WD Red Pro 6Tb and 2 WD Red Pro 8Tb
I was thinking that using SSD(s) for cache acceleration might be a good idea as might a graphics card.
Please can you advise
1. Will ssd's be beneficial for my usage
2. Is it better to use the 2 inbuilt M2 SATA slots or use a QNAP PCLe expansion which takes 2 SSD'slots and also adds 10Gb ethernet
3. What size SSD's should I use and how many
4. Is it worth adding a Video card and if so which do you recommend

Thanks
Martin