1) Yep assuming even distribution. The number of visitors per month isn't really a factor you need to consider it's the number of concurrent users. 100k evenly distributed is 2.3 per minute. You also need to consider what your app does, in your case wordpress should be fine.
2) Yep
3) Yep but you might as well upgrade your node it'll be more cost effective. $10 gets you 10GB where the extra $10 to upgrade to a 2GB would get you 24GB.
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