build suggestions Athlon A6-5400K worth adding upgrades for a light gaming PC?

[acegard]

I acquired my parents' old PC when they upgraded and am assessing the viability of making slight upgrades to make it worth keeping around for a media center PC capable of light gaming. The intent is to give it to a cousin who wants to play Fortnite and Minecraft with his friends on PC.

The weakest part of the PC is the dual-core Athlon A6-5400K which, though boosts properly to around 3.7GHz, spikes to 100% when launching Edge browser.

The motherboard has a free PCIe Gen 2 x16 slot and a six-pin PCIe power connector, so adding a GPU is an option. I am assuming that I buy all parts secondhand.

Questions:

• Is adding a GPU worth it or would it be hopelessly bottlenecked by the CPU?

• If not, what GPUs should I look at getting that would offer reasonable performance at 720p or 1080p in the main titles (Fortnite and Minecraft) over PCIe Gen 2? My main thoughts would be something like a GTX 660, 960, or 1050. 

• If I were to look at a processor upgrade, what should I look at that fits in the FM2 socket with a reasonable TDP?

Additional Information:

• Motherboard: Elitegroup ECS A75F2-A2. Not sure if it supports more that 8GB of RAM. In fact, I can't find any more information about it online or on Elitegroup's site.

• PSU: Antec 380W

Thank you for any suggestions and all of your help!