From my experience it is easy to get a judgement.It is very hard and time consuming to collect.Therefore especially on any tenant that is questionable you have to get a large security deposit to compensate.If you don't you will be sorry later.For judgements you can try the bill collector route but it almost never works,you can try an attorney but that doesn't work,you can do it yourself,or hire a judgement collector company.You have to make sure the past tenant is not "judgement proof" and will just file bankruptcy or has nothing to lose to begin with.After years you might collect some of the money.The best thing to do is either hire it off to a "judgement collection company" where they pay for court costs and collection split what they collect 50/50 with you which can take years OR you can sell the judgement to them for maybe 15 to 20 cents on the dollar and they pay you today.The judgement has to be for a certain size and they will research the debtor to see if they are judgement proof before offering the cash upfront.

the SBA discounted the lien significantly, but insisted that the balance remain as an obligation of the debtor personally.

However, if the debtor tells the collection agency NOT to call again, they can't.  

My guess is it is an agency loan (Fannie/Freddie) high LTV loan from 2005'ish - so it may have PMI.A loan modification will not have lasting negative effects since the debtor and creditor came to new acceptable terms of repayment.  

Also, the interest cost monthly - rolled into the loan and due upon satisfaction of the debt or is this, generally, an "out of pocket" expense that needs to be added onto whatever down payment they are asking for?  

The rental income  from your first property would need to be reported on two year's tax returns before it would be considered an income property (meaning a lender would no longer see it as debt or factor it into your debt to income ratio).

I'm working on a research report and looking for a reliable source for the size of the non-performing and sub-performing residential mortgage loan market in the U.S. defined by either value of debt or amount of loans.

One aspect that you may want to consider is that even thought your generally correct the debt would be taxable their is an exception under the code if the debtor is insolvent.  

Under FCRA you do have permissible purpose to pull a CR to skip "locate' debtorsThey offer the big 3, (though I recommend Experian) as well as a few proprietary credit based reports geared to skiptracing such as "Bank & Debtor Locator" "Trace Detail Report' and "Phone/Address Search" I suggest sticking with the traditional credit reports and not using these just mentioned as the information is more than likely not going to lead you to the debtor.  

Just make sure you have a sound exit strategy when loading up on CC debt, or any debt as a matter of fact.