Here are the details:9 Units in Apartment (5 2bed, 4 1 bed)1- 2 bed house on property - it is a strange house. 

I'm really excited to be a part of this community and hope to add value and learn.We just put our first offer in on a rehab home...hope to hear this morning if we got it.

We own a 12  unit- 1 bedroom apartment complex.

Is the Rent you are currently paying for a House and or an Apartment, factored into the DTI calculation as a monthly " Expense " ? 

Do you keep the inspection, appraisal and financing contingencies?

And I believe they are willing to sign a 3 month lease until they find a new apartment elsewhere.

Always have an Inspection contingency clause in your offers to let you to walk away if you find out any repairs you did not know about without any penalty. 

If you put it under contract have it inspected to determine the current condition and life expectancy of all major components and appliances.  

.- I haven't gone through my actual due diligence period yet BUT I will have an inspector go through and I will then get my actual estimates off that inspection.

You could also let each new property pay back the money you borrowed against this one to eventually pay the HELOC back down.If you are looking to really go for it and get into commercial/ apartments I would sell this and go for the 1031 as suggested by @Caleb Heimsoth.