I am getting Rich Dad, poor Dad right now.What I meant by looking at REI as a retirement investment, was that we are thinking to have a couple of properties when we retire to supplement our 401Ks and such, when we retire at 60+yo Vs having a bunch of properties for monthly income.

Looking for $40-60k Single family homes to purchase (that requires low to moderate renovation, depending on the price), paying it off before purchasing another, with a goal of buying 1 property per year to startI want to build a passive income portfolio to supplement my income, buy enough rentals to replace my income and hopefully retire early

You can sometimes get a mezzanine or a supplemental loan that is subordinate to the first mortgage, however the rates will be higher than the first mortgage. 

I will be doing some extensive work and not much of the old will remain.I have several alternatives that I'm looking at radiant floor heating, electric heat pump with gas furnace, geothermal heat pump and solar panels to supplement electric or gas heat.I would welcome other ideas, thoughts, pros, cons, etc.

Most big REO offices require a cover page and some other BS.MLS systems have a section for supplemental documents that agents are supposed to download and send with the offer if they exist.

Many of us service investors all over the U.S. and we supplement meetings with various technology solutions.

Here is a list of a few that I find to be of most value:Population TotalHome ValuesHousehold IncomesPoverty RatePopulation AgeRental Vacancy Rate*Homeowner Vacancy Rate*Educational Attainment Rate (High School/GED & Bachelors)Number of Housing UnitRent to Income Ratio*Rent to Price Ratio*Population on SNAPS (supplemental nutrition assistance program) percentageProperty Tax Rate*Median Age of BuildingsNumber of Structures by Units (SFR, Duplex, Triplex, Quadplex, etc...)Median Rents by Number of BedroomsPopulation Change (Births, Deaths, In-Migrations, Immigrants)*note that some of the metrics are composites that are computed by comparing two or more variables from the ACSA number of the variables above can also be broken up into cohorts, which will give you even more insight into how an area is doing.

I plan on investing to supplement my income.

The best I could come up with, given our situation, was buying 10-15 rentals using debt (about halfway to goal), then selling half at a future date, pay off most of the other ones with the profit, and be mortgage free with rental income supplementing retirement, so I understand the basic idea of eventually having no debt, just not sure how someone who can't just pay cash when a good deal presents itself would build up a real estate portfolio without any debt involved.

They are definitely not a waste of time IMO.Having said that, I have found Realtor.com to be the most useful with zillow being supplemental and I rarely look at Trulia.