What makes a good affiliate program?

I would like to utilize affiliate marketing for my next product (SaaS) and have zero experience.

Affiliate marketers,

• What do you need in terms of documentation?
• What kind of resources/materials do you need? (Logos, Videos, ...)
• How do you decide if the monetary side fits?
• What are the advantages/drawbacks of an established affiliate network vs. a custom program?
• Are there any red flags that stop you from participating in a program?