I signed up as my company, but I’m pretty sure that either sole proprietor or company enrolment is fine for notarising. You will need your D.U.N.S number though.
I only had my Apple ID verified and completed end of last week so I haven’t tried notarising anything yet (still working out how to even do it 😂)
If you are unsure about anything though, Apple support were really quick getting back to me when I had questions and they helped me get everything sorted with my enrolment. I just used the contact form on the website. They assigned me an assistant that emailed me everyday and even rang me at one point until I had my account setup. I was pleasantly surprised with the quick support, considering a lot of other companies during the pandemic have only had limited staff on customer service jobs.