There is no defined system requirements that will say you - ‘you need 1GB of RAM’. You need to test it and see how it works.
The thing you need to look at is RAM. CPU is not a problem, but RAM is biggest problem with Minecraft servers.

In my experience 2 GB would be enough to run 10 player vanilla server. Even maybe more then enough.
But problem is mods that can consume a lot and it could be that you’ll need 4 GB RAM plan (this is $40).
You can try to going with 2 GB, with addition of swap if needed. Problem is that swap can slow down server actually but as servers on SSD it can be less noticeable.

As I said, without test run, no one can say how much you’ll need. :)