Best option? Stick to what you currently have, including GPU. GTX 1060 3GB GPU does fine at 1080p with high/ultra settings while giving solid 60+ FPS. I know this first hand since i too have GTX 1060 3GB GPU (MSI Gaming X 3G series) and 1080p monitor (my Skylake build).
Though, if you want to still upgrade your GPU, do it after you've bought 1080p monitor, so you can see for yourself if GPU upgrade would be necessary at all. By then, you might have enough money saved so that you can afford to buy 650W PSU as well.
Btw, just because someone sells GTX 1080 with a price that is too good to be true, doesn't mean that you have to buy it when you actually doesn't have a need for it. I suspect that this GTX 1080 might have been in a mining rig which wears GPUs down up to the point of near failure. Also, you won't get any warranty when you buy used GPU, so when the GTX 1080 GPU dies on you, you're left high and dry.
Oh, PC upgrade wise, what i'd do would be buying 2nd 8GB 3000 Mhz stick with same make, model and part number since:
* latest games can easily use up to 12GB of RAM
* and you'll get much faster dual-channel RAM with 2x sticks rather than using single-channel RAM with 1x stick;
comparison as well:
https://ram.userbenchmark.com/Compare/HyperX-Fury-DDR4-2133-C14-1x8GB-vs-HyperX-Fury-DDR4-2133-C14-2x8GB/3555vs3552
Note that both RAM sets are 8GB total with speed of 2133 Mhz, while only diff is the sticks count, which makes a huge difference.
Also, look towards SSD as being OS drive since compared to the HDD, SSD have considerably faster read/write speeds. Using SSD as OS drive will give you far shorter boot-up, game loading and shut down times,
comparison:
https://www.youtube.com/watch?v=j84eEjP-RL4
For good SSD, you can go either with Crucial MX500 or Samsung 860 Evo, 500GB in size, for plenty of space for OS and played games,
comparison:
https://ssd.userbenchmark.com/Compare/Crucial-MX500-500GB-vs-Samsung-860-Evo-500GB/m418385vsm428560
pcpp: https://pcpartpicker.com/products/compare/ft8j4D,6yKcCJ/
In my Haswell build, i have Samsung 850 Evo 500GB as OS drive and in my Skylake build, i have Crucial MX500 1TB as storage drive, while OS drive is Samsung 960 Evo 500GB M.2 NVMe SSD. My AMD build also has SSD as OS drive but it has old Kingston HyperX 3K 240GB SSD.