Take what I say with a grain of salt. I am not an expert at computers.
I think you should be fine. May struggle if you try VR.
On the RAM I believe it depends on your slots on your motherboard. Typical if you have 16gb you have two 8gb ram modules (not always), if you upgrade you would buy two more 8gb modules if you have two empty slots available. its best to buy the same brand and speed (megahertz). Its usually an easy upgrade. One other option is to buy two new 16gb and replace the one you have. Search YouTube on upgrading RAM and you will find a lot of info on the proper way to do it.
On the 120gb Free Hard Disk Space, its just free space. Although I have a SSD dedicated to DCS World and one dedicated to Open Beta.
A new GPU would help. I started with a 1080 which had 8gb and it ran pretty good on medium settings with VR. I currently have a 1080ti which runs really good with VR medium to some high settings.
I don't do multiplayer (although would love to start) so I can't recommend internet speed.
I hope this helps.