This is essentially a ‘mortgage assignment’ transaction.
It borders on malpractice, as an investor, to pull off one of these deals, unless you have a plan in place, where the original seller (former title holder) has recourse built into to the contract to repossess the title, the property, and cover any unpaid payments, in the event of the ‘assignee’s’ default.
If not, it doesn’t make much difference what kind of contract you use, because the original seller is eventually going to get screwed anyway.
Putting two amateurs together, taking your fee, and letting them fend for themselves during a default, will eventually cause everyone to get regulated six ways from Sunday.
Why don’t you just set up a lease with an option? This is much simpler to remedy in the event of a default, and simply involves an eviction of the defaulted tenant/buyer if things go south.