As an alternative you can raise/level/stabilize a house with expanding foam, the downside is they will drill holes all throughout the bottom floor to inject it. 

Not sure if that will be the case but the money being injected is substantial.

This doesn't have to be your money but banks will want this equity injection up front. 

It gets more complicated when cash is injected in the purchase of that note, there is now a cash amount advanced toward that sale price, the seller of that note agrees to accept X amount for the equity represented.

Perhaps a performance bond, taken out by the buyer's contractor would have been appropriate condition of the sale to inject a level of protection for the seller.   

While you and your wife can help her father invest his capital, you are disqualified parties to his IRA and cannot benefit from or inject value into the IRA through your own efforts.

I will do a refi on that building when I decide I need another injection of cash for another round of purchases.  

If your house is of this construction, there is no practical way to inject/blow insulation into the wall cavity and you are left with the choice of either gutting back to the studs on the inside or removing the exterior cladding and applying housewrap and rigid insulation on the outside.If you do go the interior route, be prepared to encounter hand-sawn timber with substantial variance in dimensions - meaning you will need to shim and plane, or strap, to create a plumb wall for hanging sheet rock {putting back lath-n-plaster would be cost prohibitive and, in our case (a rental), non-compliant with current fire code}.Old buildings can be rejuvenated and made much more efficient, just factor in an additional 10-15% in your reno budget to address any discoveries.

I worked for years in manufacturing, (injection molding) specialized to a point.

Many seem to inject different sciences to evolve into some logical process but take care not to mix principals of economics, finance and accounting, which is what I told my students teaching economics, accounting and finance in college years ago.