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.

My plan is to create a very generic version of this to keep copies of in my car to simply leave in the mailboxes of vacant properties I see.  

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.

A quick Google search shows:The average rent for a studio apartment is $1,604The average rent for a 1-bedroom apartment is $1,868The average rent for a 2-bedroom apartment is $1,994The average rent for a 3-bedroom apartment is $2,126These are generic numbers for apartments.

That would keep the funding monies from being considered profit.So figuring this out from what I've read thus far, I would need to charge an interest rate on the loan (perhaps something nominal like 1% or 0.1%), work up a generic loan document that would state the terms and such. 

It was a specific postcard mailer that will be followed with generic postcards.

Ultimately, we couldn't come to any agreement with this production company (let's just say there were "creative differences" -- in other words, we refuse to do just another generic flipping show like they put together in the sizzle reel), but now that our agreement with them has expired, we're talking with a couple other companies.

Also, I'm a big fan of copy/paste, so have your generic email with all the information (price/qualification/showing date) and copy/paste to everyone you get, it's quick/easy and efficient!   

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.