More people will sell their stock before they sell their house.  

---So, get a GREAT property inspector NOT recommended by the turnkey company to avoid bias.2) Misrepresenting Property Class - have seen many turnkey providers in Metro Detroit area sell OOS investors Class C properties as Class B.

right now I have a property that is in need of a lot of rehab. The roof is caving in and a lot more. I’m not seeing any comparables listed or sold in the area recently in this bad shape. the last lot I posted for sale...

I hold land personally with my wife that I am selling and at the same time I am purchasing my first str.

HMLs are ideal for value-add strategies, where investors purchase, rehab, and then reappraise the property to pull out equity and reinvest, or to sell the property. 

Research comparable properties in the area to gauge pricing, rental rates, vacancy rates, and demand.Property Insurance: Obtain quotes for property insurance to understand the potential costs and coverage options.Survey: Consider getting a property survey to confirm boundaries, easements, and encroachments.Due Diligence Contingencies: Include due diligence contingencies in your purchase agreement to allow time for inspections, assessments, and resolution of any issues uncovered during the process.Risk Assessment: Identify and assess potential risks associated with the property, such as market volatility, tenant turnover, maintenance costs, or regulatory changes.Exit Strategy: Develop a contingency plan or exit strategy in case the investment doesn't meet your expectations or unforeseen circumstances arise.By thoroughly conducting due diligence, you can minimize risks and make an informed decision about purchasing the property.

Do we, for example, sell our current SFR and put all/most of the equity into a down payment on the duplex and have a lower monthly mortgage payment?

For a construction(or bridge) loan, most lenders would want 2-3 projects(buy, rehab, hold/sell, construction) experience usually in the last 3 years minimum.

If they were to sell it on the market at a higher price and interest rate the numbers would not pencil out for me to rent it out. 

Purchase a property at least as much as you sell.