What are the other strategies exists?

I would recommend joining local facebook groups of hosts (if they exist) for markets you are trying to get in to. 

We want to do this transaction so we can upgrade our living space, have another rental (our existing house), and eventually rent out the new house (his house) after we grow out of it.

This allows you to access funds while keeping your existing mortgages intact.Second Mortgage: Explore lenders who offer second-position loans on investment properties, though rates will be higher.Cash-Out HELOC : While traditional banks often restrict HELOCs on non-owner-occupied properties, some portfolio or private lenders may offer HELOCs for investors.With $15-20K in liquid funds, look for deals where you can negotiate terms:Seller Financing: Negotiate lower down payments or interest-only periods.Subject-To Financing: Assume the seller’s existing mortgage while covering the down payment.Lease-to-Own: Lock in the purchase price while using rental income to build equity.

However, if you plan on living in your existing house and when you convert the basement you can rent out for an additional $1,500 then your cash on cash return of your $50,0000 would be 36% (18,000/50,000), which is a great ROI.

I hate to be cynical but I feel the same way you do, I need things in writing.

Portfolio loan against existing assets for immediate payment to investors.

Do 80/90% of land 100% of construction loans exist?

At that time, few realized that you can buy and sell loans, so the software didn't exist.

I wasn’t clear with this strategy tho if the seller would have to own the property outright or they could still do it if they have an existing mortgage on it?