Housing purchase with balace on National home mortgage?

Hi I just want ask on housing purchase, my friend is selling their house on me for 600k less the balance of 300k- so i will buy it for 300k and will continue the remaining balance,

what docs or certificate should i require from them so that every transaction would be a legal transaction
or any other tips that i should do on making this purchase

thanks

Read more: Housing purchase with balace on National home mortgage?