They don't have the cash to do a normal down payment.

Is that investing strategy about buying the mortgage on a property and collecting the payments as income or am I misunderstanding?

Here's some options I was considering:-put a down payment on a piece of land for a future home build for ourselves.

I am not an expert by any means,  but I do have rentals in south seattle and burien; near renton and I have looked into this a couple times when I had sec8 recipients inquiring about vacancies.The way that the housing authorities require you to verify income is that you can only verify income against the tenant's portion of the overall housing payment.  

My holding costs would be my mortgage payment ($608) plus utilities per month correct?

Payments of rent via ACH deposit within 5 days. 

Thus...usually for our flips, Demo starts fist... so we'll pay in this manner:DEMO BUDGET: $6000- to start: 33%: $2k- progress payment: 33%: $2k- complete: 33%: $2kFLOOR INSTALL: $8000- to start: $2600- progress: 2600- complete: 2800this way you eliminate any grey areas, and usually contractors save so much for the end, and if you pay against the total, the budget will go through the roof!

Down payments aren't a concern. 

You will not own these properties within your TSP, but it can give you the seed money for a down payment. 

My credit score is great (770), I have enough for another down payment (20%) but lenders still can’t lend because I have around 230k in debt from the other home.