Help Needed - new MoBo/CPU/memory

Hi,

I need some advice on a new motherboard/CPU/memory combo. I have been using Unraid for a few years now solely as a file server to store movies and I run Plex on a Mac mini.

My Current build is 

MoBo - ASUS E45M1-I (Mini-ITX)

CPU - AMD Duel-Core E-450 (Integrated)

Memory - 8GB DDR3 (2x GB)

Storage - 7x HDD (6TB Parity & 22TB Storage)

I want to achieve a few things. I want to expand from 7xHDD to 12xHDD. not sure if I need that to be all storage, This may include a cache drive or an extra parity drive. The second thing I want to achieve is to move my Plex Server from my mac mini to the Unraid NAS.

Motherboard -  ASRock Intel H370M Pro4 Coffee Lake Micro ATX. I thought this would be a good option as it has 6x onboard SATA ports, it also has 2x PCI-e x16 slots. Reasonable price at £80

CPU - i5 Coffee Lake. Is an i7 a bit overkill for what I need as they are double the price? what one should i go for 8400S, 8500S, 8600S, 8600K?

Memory - the MoBo supports 64GB (4x 16GB). I think I will only need 16GB, should I get 1x16GB or 2x8GB. not sure what is better ifsingle channel will be better for gradual upgrading.

THanks