Don’t do that. You should follow the standard install instructions and install it yourself. My https://dashboard.literatecomputing.com/ will create the droplet and do the install for you, as well as configure mailgun. If you join the free trial group and don’t need my help, you can use it for free.
That said, I recommend the 2GB droplet for anyone who can afford it.
Yes, but if you try hard you can find a way to downgrade to https://help.mailgun.com/hc/en-us/articles/360048661093-How-does-PAYG-billing-work-. See also https://help.mailgun.com/hc/en-us/articles/203068914-What-Are-the-Differences-Between-the-Free-and-Flex-Plans-. They don’t make it easy to figure out how to do that, though.