The city of Portland also recently passed inclusionary zoning mandates that require any multifamily development that has over 20 units include 20% of affordable housing component. 

I like this, in that it's far easier to move factories away to another state or abroad, compared to huge healthcare facilities with significant research components, and which serve lots of patients, both locally and from farther away.6.

You have two components: tax lien certificates, and tax deed sales.

It sounds like your area is competitive like mine is and I've found that getting yourself noticed IS a major component to getting your prospect to open, call and remember you.  

But my partner said his mentor mentioned a more emotional sentimental component on handwriting the letters.

Although cash flow is important, it's only one component of returns.

Cash flow is one component of returns but appreciation over time is key to creating long term wealth and should not be ignored. 

The "rental property process" includes a lot of different things, from all of the components that @Austen Mueller mentioned to things as basic as your current financial situation. 

You'll just give them the opportunity to create something with XYZ while they neglect every other component you didn't mention.

Consider that if you build your portfolio to 10+ SFH, and you're 100% occupied; each tenant causes damage to a deemed essential component of the house, requiring professional repair each month but they're "excellent and don't complain", would you pay for the repairs, not holding them accountable?