I am going to run an individual experiment (no WaitPage) whose database has 740 vars X 1200 lines (subjects), and 274 pages (of which each subject should see around 110 in rapid succession). The deployment will have many students accessing the link simultaneously by the hundreds.

The high number of vars is due to a 60-round app that collects some vars at some periods but not others.

Is Postgres Standard-0 with Dyno Standard-2X enough provision for the expected bandwidth, more or less?

I expect it would be OK, but keep an eye on your metrics in your Heroku dashboard. There is a section for your postgres database.