Looking for suggestions to option or buy and lease back

Hi I have apartment building and triplex and would like to buy adjoining property to build more. The owner has said they would sell to me. Elderly mother has moved in with them and they need to stay in the (needs tore down)house until she can get in a home. They would then move to an area where they can buy a house for 50 to 60k. They have a reasonable mortgage pmt and reduced tax bill for low income. They would sell me an option but need to stay until mom moves to home or can’t be cared for there anymore. Their holding costs are less than if I buy and lease it back.
Purchase price 150k
Mortgage 112k
PITI $850 mo
Market rent $1000 mo

Property would allow 6 units at $1100 a month each

Any suggestions would be appreciated. Thanks

I am new with forums, Maybe I should be more clear in my question.

For now the owners need to stay in this home. Finding them another place where they can have enough room for mother won’t work in this area. Housing is tight. How do I structure this and get an option indefinitely? When they are ready to move, I would purchase the property tear down the house and build 6-7 units that would gross $6600 monthly.I am a builder and can do the work. Owning the adjacent rentals would be an advantage for management etc.
I am thinking of something like this,
$1500 fee for option with indefinite time.
When they decide to sell I have 60 days to close paying the remaining 148,500.

I would want them to be responsible for the property until I have possession.

Any Ideas?