Most people that I’ve had the opportunity to do business with use the percent of property value because it ties directly to the cost to repair the physical asset, whereas rental income can vary significantly in relation to the cost of repairing the property.

one word if you want to grow and you do not have the physical capital... leverage.  

Since, I want to live in a Quadplex or Triplex, safety and well being first is a concern.

Should I attend a physical real estate school or should I take online classes?

You can usually ballpark a starting point for your offer by assuming 50% expenses as long as you have a rent roll, and then re-negotiate as you move through your negotiations and discover more about the property's physical condition and financial performance. 

Has anyone seen success from online advertising, or do  physical yellow letters increase the perceived legitimacy of one's investing operation to a prospective buyer?

I prefer to lose a client rather than my license..Well, with this attitude you may very well be "sitting in your office" a lot.

It's a business, if you find some is abusing it financially (not paying rent) or physically (damages),, you push the envelope and write up a lease violation and if necessary evict.. doing regular inspections let's you bill tenant for undue damages when they occur.. and yet still have the deposit left to help when they leave.If you haven't yet,, download copy of the landlord tenant laws for your state.. 

Make sure your PM has experience so that your portfolio and financial well being isn't jeopardized by their learning curve.Best of luck to you!

If they see the condition improving, they may well be happy to pay a bit more OR decide to move on their own.It won't necessarily be an easy process, but you have a lot of options.