I've found FCI Exchange and LoanMLS to be good resources to look at notes. 

Once you're ready to put your feet up, 1031 exchange those SFR rentals into high cashflow multifamilies, apartment buildings, trailer parks, etc.  

Another thing to note is that if you form your LLC in state "A" and then want to do business in state "B", you will probably have to foreign file in state "B" in order to do business in that state.  

You could also looks at a 1031 exchange if you feel you have opportunities you can jump on in a timely matter.

Once you complete your DD and work out a price it is a matter of signing a contract and exchanging your money.  

@Richard CheungMy experience is that you can obtain an foreign investors mortgage through a mortgage broker.

Am I personally comfortable retiring 5 years later than I otherwise could in exchange for going to Europe, South America, and Asia in the (hopefully) first 1/3 of my life?  

If however you want to or intend to take legal action on the notes (ie. foreclose or collect), you should register as a foreign LLC with the foreign state's secretary of state as you would be engaged in business outside of the LLC's state (probably the state where the property, and possibly the borrower are located).

I have realtor friends that let me use the MLS, and in exchange, I package my findings and report back to them regularly.  

In exchange for replacing your name on the contract, you agreed to receive a fee.The buyer and seller both have the right to change the purchase agreement.