That way, sons pay for the down payment, make the payments, parents are the residents, and sons acquire ownership in the future.

You have a problem right now and the time to address it is right now.In an ideal world your tenant with the late payment record would progress to nonpayment and you could immediately evict for that.

How much are the monthly payments?

Probably not enough for a down payment on my own. 

This will require capital to start purchasing vacant land, but it would be significantly smaller than a down payment on a house.  

For example, if I buy a group of payments; do I have to wait for those payment numbers to arrive before I get paid back for them?

It's rented now at $1550 as well, on a $540 mortgage payment (5.85% 5/20 ARM.)I estimate a net return of $5K yearly on each property after expenses (thanks, BP calculator!)

Your profits will also depend on your monthly mortgage payments.

Rentometer is still free and it's generally reliable (at least here in metro Atlanta).

Here you can turn one year's tax return (or one year's contribution to a self-directed IRA) into a down payment on a new rental property.