Others have hit it on the head.

If they are available online or in person.

@Dhruv Patel I believe you are getting ahead of yourself....Where in the US are you all-in (land acquisition, soft costs, hard construction) on new construction multi-family for $100/ft.

Do they have any reviews on Google?

However, evaluating your sentiment (and your wife's) on that will be important. 

We plan on living in the house and doing most of the fixes and renovations ourselves.

Try to dispassionately evaluate every opportunity with a goal of maximizing after-tax returns net of risk in line with your own goals and tolerances.  

I've got about $525K in equity in the property, but lenders have told me that I cannot get a HELOC to access that equity for additional purchases because the duplex was financed as an investment - they have said TX law dictates that HELOC can only be used on primary residences.

They are highly secretive however - no online presence under their names while being very tech-advanced, they also installed like seven cameras around the house perimeter.

Here are a few suggestions on how you can maximize the wealth of information available: Check out the blogs and podcasts, or take advantage of the search feature (magnifying glass).