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.  

I definitely need to drive around these areas and see the physical conditions.

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?

So it's 500 bucks up front for new people (unless referred to me by one of my regular investors) and that covers me physically going to 10 houses.

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.. 

For SFH its squared footage / bedrooms / and area that help command price basically just like apartments.. harder to shop in person for comps so listings on major sites are easy to track by area / map Apartments basically the same way,, but transportation / bus sometimes is a big plus.. and if your in a close nitch with similar size apts I'd recommend go and physically check to see what they are offering,, shop them as I bet they have shopped you.. 

It also included taking time to physically drive by the property and checking out comps.