Unless it is a FHA loan or VA loan, it will not be assumable and a "subject to" purchase will trigger the due on sale clause of the mortgage. 

That seems to be the amount that triggers something where financial institutions have to pay attention.

I've strongly considered TK SFH to expand my investment "footprint", but haven't pulled the trigger yet as I have been seriously looking at smaller MFH opportunities (which haven't been good deals, BTW).

I am able to find a lot of great deals, just haven't pulled the trigger on any of them.  

That is what a buy-sell agreement attempts to circumvent, there are business and life occurrences that trigger state laws, such as death or bankruptcy.  

If you do end up pulling the trigger on this one, the silver lining is that you will learn a TON about the rehab process because you are going to encounter a lot of things you didn't plan on.  

I don't believe he will ever pull the trigger even though I encourage him.

Very simple to set up; can be done at closing; can't legally trigger DoS clause.

Of course, the land contract sale could trigger the due on sale clause and both parties should be aware of that and have a strategy in place if it were to occur.

I am still learning the In's and Out's of Real Estate but haven't pulled the trigger on any deals.