Buy with a credit card through one of the companies that resells grandfathered Google Apps accounts and you should be okay.
Edit: I just remembered reading an article [1] about hooking up a regular gmail account to the free tier of mailgun, so you send and receive through your custom domain but otherwise have a plain old gmail account.
[1] https://simplyian.com/2015/01/07/Hacking-GMail-to-use-custom...