FauxDragoon

The great thing about your use case when it comes to servers is a lot of the common applications aren't all that resource demanding. At least from a compute power need perspective this should do you just fine so long as UnRAID plays friendly with the hardware. The high single core performance of the CPU will suit your Minecraft server well too.
 
You only need to worry about that if the slot you use goes through the chipset but a PCI_e 2.0 x8 slot will still have more than enough bandwidth to go 10gig. If you do use a PCI_e slot that goes through the chipset you could run into a bottleneck potentially.

I'm actually using a machine very similar to yours right now, only mine has a 2600 non-K and the regular P8Z68-V/GEN3 non-Pro. It'll be way more than enough for a NAS. My NAS has a Core 2 Duo and 2 mechanical hard drives yet it's able to saturate my gigabit network. Plex should also run fine on that machine. I don't know about Nextcloud, but it should work fine. 
 
I don't imagine you'd have any issues with adding a faster NIC, but you might run into some storage bottlenecks if you go too crazy. You're lucky though, as your board has 4 SATA III ports built-in. Mine only has 2 paired with 4 SATA II ports. 
 
One thing I would recommend is pulling out any graphics cards it might have to cut down on the power consumption. The iGPU will be fine. 

short answe: yes
 
long answer: definetly, should make a nice Home Server, be aware though of the power consumption

External USB 3 HDD cage? 
OR
If you have space, put it in your external 5.25" bay, thats what I did.

I used to play a lot COD on PC and some CS. Honestly I think input lag is more important than Hz, generally though if the monitor is 120+ Hz then it will also have minimal input lag. That said there are some IPS monitors that have a lot more lag than others, one of these is the LG ultrawide you're talking about. I don't think it would be a good choice for FPS because of this and because of the aspect ratio which is rather unsupported in games from what I've heard. As if you NEED 144Hz, that's personal preference. I used to game on an 17ms lag monitor (1 frame at 60FPS) and it's definitely noticeable but no deal breaker for me. I don't feel like the lag is longer than my reaction time or that it had any severe impact on my performance. Think about it, there's no way your reaction time is that fast (have fun with an online test for example).

If you want to be good it will help. But that really comes down to reaction time. I swear by 120/144hz /lightboost. Ill never go back to 60hz again. The difference is totally worth it to me.  And yes the colors will be better on IPS but color accuracy is of no relevance when theres motion blur in fast paced shooters. Oh and with rog having g sync I imagine it being even better.I haven't tryied gsync yet but its sounds awesome :lol: