I've noticed that many lenders will give the same rate (usually with 0.125%), but will have wildly varying costs.

I decided to research and see what affluent, financially free, and wildly successful RE investors have already done for decades upon decades.

I currently live in Massachusetts and prices around these parts are wild.  

That's a big wild card.

If you look at margins nationwide California,Parts of Florida,New York,etc. are all speculative.I remember a 4 or 5 years back in Miami there would be only so many condos being built and sold in a development.The investors would pay a homeless person (no joke) 100 bucks a day to stay in the line for them.They would buy up units and then immediately maybe in 1 to 2 months flip them for almost double.The markets were totally crazy then.Same stuff in Cali where brokers were telling me trailers were going for 750,000.So some of these investors that bought and made a ton along with foreign investors are ready to play the speculation game.They are not really in it for the cash flow.They already have a nice portfolio and chunks of cash to allocate funds to speculative investments and if they don't all margin out they will be fine.Speculating is fine if you have the money to lose and still be flush with cash.So these speculative markets have very wild swings in values.First to tank down hard and first usually to start going back up.In Georgia we have more moderate peaks and dips.So with some appreciation back but not huge swings in the future you need the cash flow as part of the deal.I don't like speculation just the hard numbers.I will speculate to a degree on land for future commercial development but I won't buy a house that loses money every month just on the potential of appreciation in the future.

One of my tenants complained that wild animals (raccoons maybe) mess with his trash and that he tried everything and it didn't work.

Nevertheless, these people can give you a much better idea as to a property's market value than just you making a bunch of wild guesses without any outside input.

I suggest you become familiar with the industry that you set up any website for, rules, gegulations and law as well as ethics apply to many industries, a website doesn't make the world of business the wild west. :)

HELOCs are more of a banking product than mortgage product and so the rate and terms can vary wildly from one institution to the other.

@Eric Andersen the short term rental market in DFW market is still in the wild wild west stage.