Being a HML is incredibly complicated, and time consuming.

It does not always make sense to have the note recorded on title as it can cause other complications down the road, but that also means much less protection for the lender so it really depends on the circumstances. 

And if one pays half and nothing from the other and you need to move on to eviction it could complicate the process and increase the time it takes.

All this being said, insurance contracts are complicated and I’m by no means an attorney, just giving my .02.  

If moving suddenly complicates things, that is a risk you have to think over. 

If you have cash its not hard to analyze a property and get the loan. when you don't have cash then that is where you have to do complicated things to pay for a property that will be profitable

All of this is nuanced & complicated which is why I don't believe any single calculator exists. 

It's not impossible to do a 1031 exchange out of a property while carrying back a note, but it's complicated enough -- if you're going to make the deal attractive to him, find out if he plans on looking at a 1031 and come prepared with information to help him navigate the hurdles that come with the seller financing. 

Much less complicated / expensive than building from the ground up. 

Nevertheless, it will be a bit complicated.