Eliminate debt, establish a budget, and save.

Waiting until you get a deal under contract to then see if you can put together the debt and equity is a recipe for disaster.

(income = 3 x monthly rent, Debt-To-Income, etc. can still be used!)

Finding the correct contact info is hard enough, but if your pitch is that they should sell cheap now, you're misreading the market.Multifamily properties are worth significantly less now than they were just a few years ago, and asking for a further discount due to property condition would probably result in a loss, maybe not even pay off the debt.

The challenge we're facing is the lack of short term rental data for the appraiser, and we won't know if it will meet the debt service coverage ratio (DSCR) requirements until the appraisal is complete.Has anyone dealt with a similar situation?

Two main reasons being: (1) debt does not increase your basis in an S Corp, which means your losses get limited.

Key Check gave me credit and debt load, which was surprising, as most other methods people don't share.

In this case even if the project went south and you broke even or even lost money, the equity (debt) you used would still be covered by the income you are earning on the rental property. 

You would only pay taxes on all passive investments in a traditional SDIRA at distribution, unless you have a mortgage, then the portion of the income derived because of the mortgage will be subject to UDFI (unrelated debt finance income) which will trigger UBIT (unrelated business income tax). 

Purchasing subject to allows you to (1) buy a property and pay a lower rate (3-4%) that was in existence when the loan was originated, so cash flow will be better as well as equity buildup; (2) not have to qualify for the mortgage saving time, expense, and allowing property purchases in greater number than otherwise and (3) no personal liability on downside (4) no debt added to your PFS.