Pardot is the easiest to learn and then execute. Will take your team the least amount of time to provide managed email marketing & automation services to your client. Naturally you'll want to bill fixed rates.

White label email automation is an oxymoron.
Don't stop taking massive action.
Best of Luck,
Michael T. Irvin
michaelirvin.net
My books are available exclusively through Amazon Books. Check out my book "Copywriting Blackbook of Secrets"
Copywriting, Startups, Internet Entrepreneur, Online Marketing, Making Money

Have you looked into Sharpspring? It is a pretty powerful solution geared towards agencies who are looking to white label marketing automation. They sell licenses to agencies with substantial markup in mind, too.

If you have a couple bucks to spend, Infusionsoft or Hubspot. You will get out what you put in.

I would recommend sendinblue because it has feature that you would need for instance excellent transactional emails and advanced contact segmentation.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath