If your i5 bottlenecks your 1080 means your i5 is not capable of getting the most out of your gpu.your performance will get worse.
2. If you want just gaming and save some money go with gtx 1070.If you do video editing and want to spend more money go with a i7 6700k
3.SSD improves performance.It improves overall speed.make pc boot faster. Should be in your list.
4.Now if that was my build i would have gone for A asus motherboard. For the build quality and better UEFI.But it depends on you.You like gigabyte? Match with you color scheme? Then go for it.Gigabyte is good as well.
4.Monitor depends on your budget.
If you want a high end monitor
Asus PG279q for the most out of gaming.
Acer X34 for the ultrawide beauty,premium color quality and its also good for gaming.
Both have G-sync