I think this would serve me well as I will not have a large amount of cash for a down payment and would be using an FHA loan to finance this first deal. 

You should still do credit and eviction check.The 3 X rule is for the average person with car payments, credit cards, etc.  

I plugged one of these (https://goo.gl/aMtM7K) into the rental analysis calculator here and it would require a $73,000 down payment.

I am amazed at these construction costs - I would be utterly shocked if someone could build anything right now for 85 a foot, unless of course you are the GC and not taking any payment for your labor.. in which case you are still subsidizing a weak deal.

Have the seller finance 10-15% of the down payment.

Commercial (large apartments) are my end-goal, but I don't really have the cash for a down payment on a commercial loan right now.

Credit score is 815, never missed a payment. 

You could use the IRA for the down payment if you change it to a self directed IRA or take the penalty.

The tenant must be held liable for any and all late fees.In the case of auto payment no excuses for not payment should be accepted and no leniency should be shown for late payments.Be very aware that payments will immediately stop in the event the tenant should become unemployed.

I intend to use equity on existing property(which will be a rental) as a down payment on the next property(ies).