I started at age 20, dead broke with no experience but hungry with lots of time on my hands. 

I think another factor here in determining whether its a good investment is your age and your goals. 

Given their age and position in life… mailbox money flow forever sounds like a good start to retirement to me!

For us approaching retirement age, we want to have a mix of mortgaged properties and properties with full equity to leverage instead of relying on saving up for the next investment. 

For a rule of thumb, replacing a commercial boiler can range anywhere from $20k to $150k, depending on the size, age, and efficiency.

Quote from @Dennis Bragg: Hey Josh, thanks for jumping in with such a bold approach.. it’s not every day you see someone your age pushing for a luxury twist on a single family rental.

BTW there is a free trial and the Starter monthly plan is $119.It also allows you to search through consumer information such as property owner "credit worthiness," Credit card usage, buying power, age, language, family members in the house, and many other fields that are above and beyond property characteristics, which is unique.

My main goal is it be able to retire at age 40 from my main job and work entirely on my real estate portfolio.

I thought SDIRA max contribution is $5,500 or $6,500 if over the age of 55?  

The biggest factors, from what I can tell, are age and rebuild cost.