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I would say a normal 2GB droplet for $10/mo would be sufficient for your needs. Since Gitlab needs around 1GB RAM + swap as well and you got around 1GB of other apps, this would work.
Even tho, if this isn’t working properly you can always easily upgrade your droplet. Downgrading will be harder since you would need to make a new droplet. So starting at the bottom is the best for you I would suggest.
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