Assuming they know how to code emails properly (i.e. "design like it's 1999"), I can't think of any special considerations specific to Eloqua. If they give you well-coded HTML you should be able to upload it to Eloqua, and you can edit it in the nice new split-screen code view which works a lot better than it used to.
I think the main concerns I'd emphasize are responsiveness and cross-browser compatibility. Litmus is a good tool for testing the latter.