BRRRR mixed with seller financing question

[Austin Hoxie]

Hey Friends - I have reached out to a few rental properties in my area and am in discussion with two different owners about purchasing their homes via seller financing.  

1. Is a recent widow who has no mortgage

2. the other is a large trust that has been sitting vacant for 4 years after the owner (massive hoarder) died...stuff still in the house...  

Both are in my neighborhood and I am looking to turn my current property into a rental.  

My question is if I was to get into a seller financed deal with one of these two properties - would I be able to move into either of these and do a 2 or 3 year live in rehab in theory getting a significant bump in the value - then go to a bank get a mortgage and pay off the original seller? Or would I be able to go to a bank and get a HELOC - pull out $$ and turn it into a rental?

Essentially trying a longer live in BRRRR - and I am not sure what type of legalities play into seller financing.

thanks for any help

(I got denied by a few banks to get mortgage's on either of these two properties, hence the seller financing)