Getting started - Assumable Mortgage Question

[Daniel Rothra]

I'm hoping to make an offer to a retiring friend who has a house to sell.  I would like to create an offer that would let me delay creating a mortgage and some type of annual or semi-annual payment for 3 years.  

I wondered if it would be worth it to try and assume their loan.  I need to find out if their current mortgage would be assumable and if that pathway would be better than the going rate.  

How could I structure a deal that would sound acceptable....   

Houses around them have sold for $400,000.  They are friends in my church.  looks like they have a lot of equity ($250,000 or more) and a recently refinanced mortgage of $120,000.  

I've thought about offering $20,000 every 6 months for 3 years and a balloon mortgage to cover the remainder...  or some lump sum and then semi-annual payments for 3 years...   I'm new at this... 

Their house is 2400 sqft; 4 bedroom, 3 bath with space in basement that could be an apartment or be used as a nice Airbnb; it's in a development with an HOA. Their lot is small; fenced in in the back yard. Two garage doors (one open area inside).

Any suggestions on structuring a deal here?

Dan