Hi Hannah,
Similar to Eli's response, if the company is familiar with designing emails, they will most likely know most or all of the best practices when it comes to email design/development. I can't think of any Eloqua-specific things that they would need to be aware of when coding.
If the company isn't familiar with email design/development, here are a few good resources.
http://templates.mailchimp.com/development/css/client-specific-styles/
https://www.campaignmonitor.com/guides/coding/guidelines/
If the email is responsive, we can provide other resources.
Thanks,
E