$521,000 cash flow annually - how to start REI?

[Josiah Sia]

EDIT: I DO NOT HAVE $521,000 cash flow annually extra to use. That was a typo! I meant to put $51,000. So sorry!! I cannot edit the post title :(

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My wife and I are blessed enough to be able to have a great cash flow of $51,000 extra a year with excellent above average credit after all our regular expenses (including our home).

What is the best way to get into REI with this extra? I've seen so many articles about starting with $0, and I know I can just follow those steps too but because of the cash flows at our disposal, is there a better starting point?

I work in IT so my best example is like if you have $500, and you want me to build you a PC from nothing, I can probably get you a decent budget PC built. However, if you have a budget of $800 and a decent PC I can take parts from already, I can probably make you a mid-range or higher-end PC build. I would recommend you not start from scratch and jump start to the next level with your spare parts + cash on hand.

Is starting off with duplexes/4plexes still the best route to go? (We want to go rentals and aim for high passive income long term) Should we just buy more at an accelerated rate? 

Any advice on what to do with our extra income would be much appreciated.