I would not aim for a project that size for start…
Then you are ok with a ram update only.
Sometime ahead, when you know the tricks, you could update then.
I can not often enough say it, i work on an i-2500k (factory) 560-ti and now 16GB ram and i am very happy with that.
Have a look at ramrush(freeware) and watch your ram, but never use the autoclean function, when inside UE4.
Shut down UE4 and launcher, restart, free ram.
Every time i start play, around 200MB ram are away. You can easy calculate when i have to restart, to clean ram, but even that is no problem for me.
It reminds me often enough to save, when i forgot it about a hour.
Weird is, when Skype is running, my ram is filling quicker, feels like influencing my UE4 used ram in some way.
When skype is not running, i double up the time i could work, before restart.
I am not talking, about ram taken by skype.
If i would be you, then i would start making scenes, like in the wip section.
Cave or something small, then next scene, then connect, repeat …