GF's mom offering to sell her portion of the condo

[Danier Martinez]

Hi everyone,

My GF and her mom are both paying down a condo which my GF is currently residing in. The condo was purchased about 3 years ago at 565k and my GF's mom is offering to sell me her portion of the condo for 300k (half +whatever she paid for lawyers fee's, land transfer tax etc.). The deal is great as she is selling the condo to me at yesterday's price. However, both my GF and her mom recently locked in a 3 year mortgage at 4.99%. If i purchase the property now, there are mortgage penalties that my GF and her mom would pay. Additionally, because my GF and I would both have to get new mortgages, our rates would be around 7.5%. We've already been to the bank and it's not possible for my GF's mom to the transfer her mortgage + her rate to me but her mom is offering to keep her mortgage and i would have to pay her mortgage down for the 3 years and then obtain a new mortgage. 

I was wondering if there was a way to create a contract (i was thinking rent-to-own) where i agree to pay off her mom's mortgage for the next 3 years and then purchase the property for a set amount? I've spoke to a lawyer's secretary but she advised to just use a trust - which i'm not too familiar with. 

I have never purchased a home before so if anyone else has another way of pursuing this your opinions would be greatly appreciated.