Home Server - Plex Media, Home file video music photo storage-

If just for storage and not running a dvr serviece (for either TV or Security camera) or any other cpu demanding service then the i5 is really not necessary at all. You can go with an i3 and still have all the performance you need.

8gigs is still good.

There are a few full atx cases that have 6-10 5.25" slots. For my home server I was looking at it but I ended up getting a Fractial Design Define R4 case. It has 8 internal 3.5 bays with removable brackets, sound dedeaning inside the case, plenty of room for fans and extra cooling, and very good cable management, as well as looks great for a server tower (no bright LED fans or anything else ostentatious like that). I then put 2 hot swap 5.25 bays in the top. If you feel like you wont really need to access your drives on a constant basis then this might be a better option then a full ATX with all hot-swap bays. Ohh the case also has a seperate SSD mount on the back of the motherboard tray.

For my home storage which will increase to 16 TB in the next month I am going to use a software called flexraid. It is a software raid utility that allows you to merge drives into 1 partition and also allows you to have a parity drive (similar to raid 5/6 but on just 1 drive) with JBOD instead of striped (meaning that the data for a file is stored on only one disk instead of spread accross all drives), it also allows you to have a mismtch of drive sizes. This setup will allow me to have the flexibality and redundency incase of drive failure without the cost/strict requirements or the stress on the hard drives that a true raid 5/6 creates.

For internet access to server it will likely be easier to setup a VPN to your network vs setting up an sftp/ftps connection to each individual drive.

I know its a lot of info but hope some of it helps or at least gives you ideas.

For motherboard that depends on if you will be getting a seperate raid card and how many drives you plan to have.

You have no need for a high end overclocking/gaming motherboard but depending on storage needs the z97 and some b85 boards have 8+ SATA ports on them.
Good brands are ASUS, ASROCK, Gigabyte, and MSI (in that order for my experiences).

FYI you will want all of your sata configured in AHCI mode in BIOS for both SSD drive and hot-swap ability.

8 gigs is plenty for current needs (hell 4 would do plex and file server with no issues) and no need to do more then 1600hz snce high freq ram wont make any difference for you. If you think you will be running a dvr (for either TV or IP cameras) or other heavy applications then 12-16gb would be needed.

For accessing the server from outside world I would setup a VPN connection, you can host VPN on the server and port forward from router, or if you have a dd-wrt or tomato router you can setup VPN on that. If you only have 1 network share then you could setup an ftps connection with filezilla instead. The VPN would be more non-techy friendly though as they just start connection like wifi and then browse the folders, the only trick is training them to drop the connection when done (hopefully UK has better upload speeds then majoridy of USA, i get 30 down and 5 up ).

As far as OS that depends on what your future needs are and if you are doing JBOD or RAID and if doing RAID are you looking at hardware or software.
Linux is lighter and is more trouble free for network shares, plenty of good applications but is also more of a challange if you are not used to linux.
Windows has some good software for specific purposes

I am running win 7 pro on my server due to specific software that I wanted to use for software raid, dvr for IP security cameras and automated backups.
I use xbmc instead of plex but have my database and thumbnails on ssd as well (world of difference for network access). On my server I will be switching to a software RAID with FLEXraid. This allows me to merge multiple drives (of various sizes) into one volume and have a drive dedicated to parity. It uses a little overhead but takes a while to calculate parity but the files are on individual drives and not spanned accross multipe, thus my hard drives take less beating and much less chance of damaging an additional drive if I need to rebuild array (vs a raid 5/6). I then have 2 drives in raid 1 for backup of personal files and computer images.

FYI, i also setup my backups to be safe from cryptolocker/ransomeware. My media is all safe from network attacks by making it read only from any account but servers admin account (created second user to be admin, and striped admin from default user). I then have my file/disk image backups to go to a temp directory on network share drive that has open permissions. From there the admin account will run software to transfer the new backups to the drive that only that admin account has write permission to. The file backup software I use is syncback se that has many nice tools including versioning so if a file gets changed it saves the previous version. Thus if a varient of cryptolocker did get on my network it would not have access to my backups, and if it managed to encrypt the files right before they got backed up, I would still have the saved unencrypted versions of the files as well.