You are right about the entity structuring but it's actually not too hard.

My wife and I are both employed elsewhere so we are not intending to pay ourselves from real estate investment, only looking to build a profitable portfolio.I think I can avoid self-employment tax by setting up an "S" corporation for the entity that "holds" property.

Entity paid cash for 10 properties (4 unit rentals) a few years ago and has been operating them debt-free since.

First, your IRA is a completely separate entity from you.  

In order to get the entity protections, you have to follow the corporate formalities, like separate bank accounts, not mingling funds, etc.  

In other words, do you want to focus on doing transactions or do you want to focus on creating a entity larger than yourself that has the ability to generate income independent of your full-time participation?

Regardless what way you go, I do suggest setting up a business entity to operate under from day one.

The financing only allows to LLC entity not personal so that is why I was going to get one started.

It can be much less expensive (in some states) and much less hassle than an LLC or other entity, which most people don't recommend for a single property.For $0.45 per month, why not have the extra $1M in coverage?

You can always establish entity at a later time, the main purpose for that would be asset protection.