One thing for sure though things don't go up up up forever and appreciation of 10%+ a year isn't sustainable.

I do have abundant experience in sustainable agriculture but I'm completely new to real estate investing and tax liens.

I have seen this over the years in many markets and the default rate for the note buyer is sky high.. not sustainable in my mind.. at least from the investor stand point..  

.* There is a self-sustaining requirement for 3/4-plex remaining rents, but, highly recommended!

It's a great sustainable model as leases longer than 30 days are fine.

So, we have the first location selection metric:Significant and sustained population growth, WikipediaRents Persist Throughout Your LifetimeYour rental income will only continue throughout your life if your tenants maintain similar employment as long as you live.

and then you use that money to do another BRRR and in theory you create a self sustaining model.

I have put my seed money in and at this point it needs to be self sustaining from rents and investing the money into stocks while waiting for the next deal. 

They might have made what they claim there in their best year ever, but that's not a sustainable number.

Now that we've seen some success, and are getting our feet wet (barely), we are looking to understand how the business can sustainably bring value to our lives.