Although you are not talking a Syndication, I would seek out Syndications in the type and size of REI you're interested and go through the investment process with them to see their setup. 

Real estate is a pretty efficiently priced asset class...the only reason that a seller would sell below ARV with enough of a spread for you to be in it for 75% of said ARV would be if it's a disaster and they have no other options.

I'm selling my primary home which has a $1700/mo income producing rental used for MTR/STR.

Does anyone have a letter template they are willing to share with me to send to property owners to sell? 

I’m curious as to the strategies that are recommended to ensure I’ve covered all my bases in the process.

Sell as a seller finance with 10% down and charge about 8% interest increasing the monthly income. 2.

He is recently divorced and has to sell his house, but he really doesn't want to move.

If I find a lie or fib during the confirmation stage of the vetting process, no excuses.

I want to buy the property on terms and demo, build a new house.I want to structure an owner finance deal with him where I either pay him interest only on the lot or no payments for 18 months (or until the new house completes and sells whichever is earlier). 

@Logan Rex if you are considering selling the SFR and going with a Highrise for house hacking purposes of eventually after the year or 2 and then keeping it as a rental.