SSD Cache Pcle v Sata?

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

Thanks for the coffee, this keeps us going. Many people buy TS-673 because it is easy to upgrade. Runing Plex on SSD would certainly help. Not only it will open things quicker but I have noticed improvement in transcoding too. There will be no difference from using built in M.2 or PCIe combo card. All you need is two sticks of SSD. One for read and other for write caching. Having 10GbE is something useful if you have infrastructure in place.The size depends on average file or project size you work on. Most of the cases 250-500GB is enough.The TS-673 does not feature graphics unit, so if you want to transcode, you need GPU. For direct streaming in compatible video format, you don't need it.I hope this helps.