But that service is not available to sole proprietors.

If a broker nets $100k, she would pay $15k in this SE tax as a sole proprietor (which is no entity or a disregarded single-member LLC).

Accounting for the rentals is different than accounting for say an LLC, S-Corp, or Sole proprietor business.I understand your connection to having at least one employee in order to get group health insurance. 

You can do business in your name (a sole proprietor) and file a schedule C.

There is no limit to the number of financed rental properties you can report on Schedule E and you are allowed a mortgage interest expense deduction for each one of them.Even if your rental activity is under an LLC (sole proprietor), you would still report your rental activity on Schedule E (1040) as if the LLC did not exist.

However you can buy as a sole proprietor and then add someone else on title who would not be on the loan.

I have a single member LLC (sole proprietor) and I'm wondering if I must pay taxes on an angel investment?

For your business you would decide if you are a Schedule C Sole Proprietor or Schedule E S-Corp or LLC.

The LLC is a sole proprietor LLC owned by me only.  

@Shariyf GreviousThe OP's question was a lot simpler, implying sole proprietor with no employees, or corporate structure.