5TB transfer should be more than enough to handle your Minecraft server. Usually a Minecraft server your size should not use more than 1TB data transfer per month. :)

It is impossible to calculate. It depends on how many chunks each player loads, what version you’re playing, how many reconnects, amount of achievements, amount of chat, any mods or server texture packs. Best to do an experiment and monitor it over time.

5TB should be more than enough for a minecraft server even with 100 users. As others have said, there is no way to know exactly how much bandwidth your minecraft server will use without experimentation but except in extreme cases 5TB will be plenty. Also, while there are bandwidth limits on DigitalOcean droplets, these limits are not yet being enforced and you will not be charged for bandwidth overage. This will change at some time in the future but not without a notification going out to all users first.