How to structure a subject to loan with a downpayment (house is on market)

[Lilly Fang]

I found several houses on the market with prices too high to sell. However, the seller has a large mortgage with 3% rate.

For example, a house is listed at 500k, with a 380k mortgage of 3%. Seller bought it at 480k in 2021. I would like to take over the 380k mortgage at 3% and pay the difference in cash, then I still have positive cash flow when renting it out. What's the cost and paperwork associated with the subject to loan besides the usual closing cost? 

I also heard if the seller one day decides to take the house back, I may lose the house? Anyway to prevent this happen? Do I also lose my downpayment if this happens?