If you do have to move because of a job, you would be safest (in my opinion, i am not giving legal advice or financial advice), i would sell that property to ensure that you did not just purchase with the goal of moving out early and taking advantage of lower rates and lower down payments for purchasing your home as a primary residence. 

The two existing LTR units (on my family’s property) were built 5–8 years ago at a cost of about $80,000 per unit, so I’m estimating a 20% increase in costs for the new builds.My plan is to use the equity in the land as the down payment for the construction loan, which I expect will make closing costs the primary upfront expense.

Consider tenant stability, including consistent payments and maintenance of the TOH.

Some gurus tell practicioners of sub-to to hide transaction from the lender by saying the lender doesn't care as long as the payments are being made.

On the lease, he'll pay me $100/m over my HELOC payment and all other expenses, including the refi cost, inspections, title work, etc... 

Quote from @Jaycee Greene: Assuming you pay asking price, I'd estimate the down payment for something like that using a hard money loan (HML) would be around 40% of the purchase price and your monthly payment would be $2,600 with an interest rate in the low 7% range (probably need a 700+ credit score to get that).

Here are some tips to keep your projects moving smoothly and scale up:Managing ContractorsControl Payments: Break the rehab into phases and pay contractors as they complete each phase.

Since it’s a new build, I was able to negotiate some great perks, like a 5.875% interest rate, no closing costs, a fridge, blinds, and even a backyard firepit.Pros:Good school districtLow interest rateMinimal CapEx and repairs (because it’s a new build)Low insuranceHigh-quality tenants (due to the school district and being a new build)Cons:Lots of new builds in the area, which could drive down prices and increase vacanciesThe Numbers:Price: $290KInterest Rate: 5.875%Down Payment: 25%Monthly Mortgage + Insurance + Taxes + HOA: $1,480 (I got really low insurance since it’s a new build and Alabama’s property taxes are low)Property Management: 10%Rent: $1,800 (this is under market because I wanted to rent it quickly—most units in the area were sitting vacant for 100+ days.

NONE of the money raised from the auction of a 2nd goes toward the payment of the 1st (that is why they call it "subject to the first")4.

The wrong tenant can lead to non-payment, evictions, and property damage.