New build - converting from 4 bay QNAP (Raid 5)

Hi,

I have a few things I need help with and would appreciate some advice on any of the point below, whatever anyone feels they can advise on I’ll be happy to hear it.

Just to give you an idea on myself, the last time I built a PC was circa 15 years ago. I've just spent the last few days deciding between unraid and truenas, unraid seems to be a better fit for me. Although I will primarily use this as a plex server (multiple users), I want something more powerful than I need right now as I need the flexibility to do what I want in the future, maybe run a VM and do some gaming etc. I could just buy a synology but I want the fun of building as well as getting back to grips with the hardware side of things, because there is an overwhelming choice of parts and I have no idea which way to go, I can't even decide on Intel or AMD which would be my first question!

I have purchased just the case so far and already have the disks. The case is a Fractal Define XL, which gives me as much room as I’d like for whatever size board or up to 18 disks. I plan on using 6TB Ironwolfs (7200) for parity (one to start off with until I feel the need to have two) the rest of the disk will be 6TB barracudas (5400) which will allow me to expand and replace drives without worrying about cost. I have between £1000 and £2000 to spend on everything excluding disks, case or a GPU (don’t need right now).

So,

1.     Is there a benefit to either Intel or AMD based on the above?

2.     Any recommendations on boards, I’d even be open to an SSI-CEB with two moderate processors if this was advised…

3.     I have an old 500w PSU. If this wouldn’t be enough what would be comfortable?

4.     At maybe 3 X 40/50GB transfers a day, would I see a benefit in 1TB cache SSD as opposed to 500GB?

5.     I’ve seen many recommendations to use a SAS card instead of onboard, would that be advised?

6.     Would an onboard raid controller be problematic as it would with truenas?

7.     Once I’ve chosen the parity disk, can I then reallocate it if need be?

8.     If I had 6 X 6TB and also 1 X 1TB and the 1TB fail, Would I need to replace it or would the data be saved on other disks assuming there was enough space?

9.     Should I use Unraid’s encryption?

I really appreciate input on any of the above or any advice relating that I may have missed. As you can tell I’m very open on which route to take so if there's any information that would be handy to know pre-build feel free to let me know as I’d be grateful to receive it.

May thanks in advance!