I'm thinking of use DO to deploy a wordpress blog (with average 400 sessions per day) that also includes one page that is actually an App powered by a node )+Mongo) backend. Is 512Mb enough power for that, assuming I can get persuade the backend to operate on a different port to Apache?
It would be a fun experiment...how important are these 400 sessions to ya? There are so many questions here, I don't think anyone can answer this without trying it first. Is there a peak time of day for these secessions? What kind of plugins are you using on WP? How complicated is your node/Mongo/App? At least you could add memory quickly here or better yet, isolate Mongo in a new dropplet and then do the same with MySql.
It generally peaks at 50 sessions per hour, but has once in the last month reached 70. The Node-Mongo piece just serves up some extra meta information (the posts are restaurant reviews, the Node bit provides the address, a rating, ... to allow additional ways of searching).