Before I buy unraid..

Hi, new here but been doing a bit or research reading forums watching youtube vids etc and getting the impression unraid is probably the best option for my use case, but have a few questions before I take the leap.

First a bit of background; Currently I'm running home assistant off a NUC running HAOS directly, also was looking at getting a separate NAS to host an SMB share so I can record live TV from a couple of TVs in my house but access the recordings off all of them, came to the conclusion best to integrate into one device, was just gonna use the NUC but its really old (4th gen i3) and only has space for 2 drives, 1x m.2 1x SATA, so wanted to upgrade.  The only issue so far is it needs to fit in a built-in media cabinet in my living room so full size ATX case is a no go, also spinning HDDs or noisy fans are something I really want to avoid too. 

Main question I have is about my planned hardware, and if it will be enough to do what I need.

Managed to pick up a cheap SFF PC (paid £81 delivered) : HP Prodesk 400 G6, 9th Gen i5 9500 with 8GB ram 256gb ssd.  I chose this specifically for the compact size, its bigger than the NUC by a fair bit but still fits the cabinet easily, it has 3 SATA ports, 1 x m.2 and also a PCI (so with a pci to dual m.2 adapter) I could eventually do upto 6 drives in a small chassis. I plan initially just to have 2 sata drives not bought yet but will be 2 x 2TB MX500s, and maybe leave the m.2 256bg in that comes with the PC but not sure if its any use? Then gradually add more SSDs if I feel I need to and if storage costs come down maybe add some 4TB version into the mix.  Understand to make sure drives have decent cache so plan was to stick to mx500 for the sata and samsung evo if I add m.2 storage.

Main hardware question is will the processor (i5-9500) be good enough as a relatively long term solution, running unraid then VM HAOS, SMB share for tv recording and then later add something like immich to replace google photos.  And is it a reasonable solution to just use SSD storage for unraid or do I really need a HDD?  I'm assuming a RAM upgrade will be advisable from the initial 8GB but it's DDR4 so it is cheapish is 16 enough or will I be better off going straight to 32GB.

I'm totally open to other options by the way the PC was cheap and I can easily flip it and get my money back, just bought it because it was such a good deal.  As I can see it the main cost is going to be the MX500s

Other things I may want to do on the device at a later point include wireguard type VPN (not done enough research into the actual app) but just some secure way to access my home network when I'm out and possibly stream media or even use it to access geo-blocked stuff when I'm travelling.  I'm also toying with the idea of a plex library stored on there but again would only be the storage as I have a couple of Nvidia Shield TV Pros so could use that as the actual plex server and that's only if I ever do bother to set it up