I'm a complete newbie interested in slowly obtaining long term properties to rent out and supplement my income when I ultimately retire (in around 20 years). 

As a new owner this is the best time to make changes.Whether you wish to collect market rent from your income property to maximise your investment or continue to supplement your tenants rent depends on the direction you want for your future.

Are you looking to invest as a means of supplementing your retirement income? 

If required, the lender can do supplements to the credit report for any updates?

My goal with real estate is not to quit my job (I love my career in medicine and plan on sticking with it), but rather to supplement my income.

Maybe try and buy an investment property in area X and rent a place in Staten Island and use the overage from your BAH to supplement the investment property. 

I had to supplement the extra $23K but it was worth it to avoid taxes on the first property.$50,000 rentalThis property we purchased for $125K with the remainder of the 1031 Exchange turned into a bigger project than expected.

My goal in starting passive income is to supplement my wife's income x2 so she can stop working and then I'll continue to work so we can continue to build.  

I graduated in 2015 with a Finance degree but went the entrepreneur route to learn new skills and build other income streams that can supplement my job and lead me ultimately to financial freedom.

Your equity is returned during equity events, i.e. supplemental loans, refinance, and primarily sale.You can certainly earn more doing your own deals, but the beauty of syndication for passive investors is just that, the passiveness.