In the beginning, I chose to try to buy the high interest high risk loans ( C-F credit grade, 15%-28%+ ).

Standard grade tiny typically sells for $30k - $50k though many people build their own.

Using age as a guide to class is more accurate with garden-style Multifamily apartment complexes and that is the context of my previous answer as it is in Multifamily and commercial where you most commonly see the letter grade system.  

Now I'm curious, assuming these rooms are below grade, are there acceptable window well parameters for egress?

Is the A-D (and sometimes F) system in common use for grading neighborhoods the best possible system for those interested in investing in low-income, high-yield areas, especially in Rust Belt and East Coast cities?

I'd be interested to know if such a listing has been pre-complied already or not, as I imagine it would be an interesting read if it contains rationale for any ranking or grades given.

The greater the equity the better the grade of investment.

If you have craigslist furniture that you have little investment in, low grade carpet, cheap appliances, etc., then you'd feel better charging less than expensive finishes, furniture, and upgrades that would cost to repair/replace if they damage them.  

I've been very successful with rentals and mid-grade flips, but I'm moving more and more toward high-end flips and multifamily.

Do cash buyers want to Do upgrades or simply builder grade fixes?