I wanted to ask if this would be considered a viable/legal deal? Or not worth risking my credit score? Or what can I do to make it legal.

Home owner has a property they owned 600k and shortsaled 2008 to a friend & family for 400k. Owner gave 70k for downpayment for the shortsaled house to friend & family. 

Owner still lives in the house but title is under friend and loan under family. 

Owner now has no means to credit the property to themselves, but wants to sell in the future. Can't sell the property now because of parents are living with owner, but may die in a few years.. So there are moving constraints for the Foreseable future. Owner has a debt settle as well with the family for 50k and is being pressured. The interest rate on remaining 300k is at 10-12%, and owner wants to refinance to a lower rate but is unable to do so since family/friend doesnt want to leverage their credit anymore. 

The owner is currently living and paying for the mortgage under the family/friends name.

Loan value is 300k, property value is 485k. 

I have the means to be able to recieve financing from the bank, at maybe 3-5% fixed interest rate. Would it be legal for me to take over the existing loan? And let say I save the owner 400-800$ a month, would it be unreasonable for me to ask for 100-300$ a month for refinancing their property. If and when they sell the property they will deed me 10-20k of the profits.  So if they hold on to the property I have passive income, if they sell sooner I get a lump some. They are willing to cover all "cost" of transfers. I'm still unsure of what may be the "cost of transfer".

I'm 25, looking to be more involved in REI in the next 1-3 years. So will this tie me down too much?

How can I best approach this deal?

-Aaron