First server!

Hey guys. I'm looking at building a Plex server, but I'm needing suggestions on a few things if thats alright?

I have been running my server on my PC which I use for gaming and everything else but its quite old now and I want to have a dedicated one thats on 24/7 and have some redundancy as I have no safety of my data at the minute. I have just started sharing the server with my family and close friends so I want to improve the funcionality and userfriendlyness of it so thats why I need 24/7 and will be looking at Sonarr/Radarr/Requestrr and so on.

I have built all of my old PC's myself so I would say I'm a beginner but know the basics so won't completely screw things up.. I hope!

I have decided on a couple of things already for it but as I'm new to the server realm I don't really know whats exactly needed or best for your buck kind of thing. 

I currently have 3x 4TB external USB HDD's that have my (TV/TV Complete/Movies) media content on and the drives will eventually be used internally in my new server in the array.

I will be buying a new 10TB HDD to use as a Parity and a new 4TB HDD to use in the array, my thinking is to format them and sort them out, then one by one transfer my current HDD's data to the array and then format and add the actual old drives to the array itself until its all done. The end goal will be to have 2x10 TB as Parity, 10x 10TB's in the Array and 2x 500GB M.2's for Plex and all other apps (used in a cash pool I think unraid calls it but I know it as RAID1?).

Hardware -

• Case: Define 7 (as it has 12 HDD bays in a tower what more could you want!?)
• CPU: i5 - 10400
• Motherboard: ASRock Z590 Steel Legend ATX (assuming I will use 1x PCIe for a SATA port card to add all my drives eventually, and maybe a 16x PCIe with a P2000 in a few years also (just realising will the CPU support a x16 and x1 being used at the same time))
• Ram: 2x 8GB DDR4-2666 (Fastest the CPU can support) will eventually double up and use 16gb as dedicated transcoding or something?
• PSU: No idea

• Flash drive for OS: Samsung Bar Plus 256GB (only going that large as its so cheap to go from 128GB to 256GB)
• HDD: IronWolf 10 TB 3.5"
• M.2: 2x Samsung 980 Pro 500gb (will the MB support 2x PCIe 4.0 for M.2 or only one, will it use 2 as 3.0?)

• GPU: I don't need a GPU yet but will be looking at the P2000 for transcoding

This will be used to run a 24/7 media server on the unraid os. Any thoughts on the components that fit with what I need? Looking around £200 for CPU, £200 for MB and then cheap for the rest. Any ideas or links to videos on how to transport my current plex with its database and so on from my windows to this new unraid?

With HDD's I'm thinking best way to go will be to initllaly buy one 10tb for parity and then a 4tb to help transfer stuff over. However, re-thinking about it, for now can I just buy 1x 4tb drive, install that and then port one drive of data over, then keep doing that till I have my new 4b and old 3x 4tb's in the array without a Parity, and then add a parity later?

I'm also now thinking about just having all 4TB's and no 10TB's as that will take ages to fill up, I'll just upgrade to 10's later and it should keep the initial costs down. But, when I do upgrade to 10TB's I will have a ton of old 4tb drives sitting in a draw doing nothing?

Anyway, its been a long read so I really appreciate it if you got this far! If you have any suggestions, tips and so on, I'd be extreemly greatful to hear them!

Thank you!