There's honestly good, general advice above, but the answer to your question is really simple. 

Granted we only have 17 but still, I’ve never used anything except a simple excel spreadsheet and all my rental properties have a separate savings account under my main account at my local bank.

So bye-bye all the Brrr's and Multi's and what not, we need SIMPLE, right, a good simple SOLID base-hit, right. 

I'm quickly learning that getting a mortgage isn't a simple task.

It’s not the most simple task unfortunately - definitely requires some technical skill.

Eric, I hope that wasn't too simple of an answer. 

You can always start with the Simple Start subscription and upgrade if you find that it isn’t meeting your needs.  

I believe that you don't get returns (failed to deliver) with bulk mailers so you don't know which addresses are bad.That's the simple stuff.My advice is to do various mailings that you or some prepares locally, that go out immediately, using 1st class postage.

The Rentastic (and other RE focused systems) do seem way more simple, and user friendly than QBO.

Remember, if a deal is truly off market, you have an owner who knows they are able to sell and is choosing not to take the very simple step of listing it.