It depends on a variety of factors: your goals, your time constraints, your available capital, your desires, your timeline, etc.

First of all I put my acceptance criteria clearly in the ad: minimum credit score (enter X amount, 620 is what I use),income 2-3x rent, good rental history, references, no evictions, acceptance dependent upon background check and employment verification, pet policy, desired lease term, etc. so that people will hopefully read that and not come to the open house if they know they won’t qualify.

There's room for flexibility depending on the desired tax strategy.Once you have more than a few properties, you'll want to consider software.

First he needs to decide what he needs and what he desires. 

Do you have the time/desire to be a PM for others? 

Compare those owners with younger owners who have higher incomes and more desire to build large reserves for the future with the older folks who won't see the future and you can see the problem.

On my end, I would never purchase a property that won't produce at lease 1k in net cash flow, and it must be in a desirable area that is expected to appreciate.

I put in my lease terms that I'll be asking the guest 30 days before their lease is due to expire whether they want to extend (IF they've been a desirable guest) and give them first dibs.

I don't have the time to study Bitcoin nor do I have the time or desire to study any individual company thus I invest my traditional retirement savings into low cost index funds.That being said, I do feel like I can study and become educated about my local residential real estate market and it is an interesting topic to me so that is where I spend my time, effort, and excess capital.Regarding, investment vs gambling. 

My tip would be to buy properties in desirable vs "affordable" areas.