K.S.
Private mortgages are basically unheard of these days. With that being said, you're better off trying to renegotiate your lease to try to stay in the property.
A little something about your "rent credits". The ONLY time rent credit can be applied towards down payment and closing costs is if you have evidence you've paid that money to the landlord (usually needs to be documented through a title company) AND the amount that is being credit back from the seller exceeds the market rent for your home which is determined by an appraisal. For example, if you pay $1000.00 rent per month and the appraiser determines the market rent for your home is $900.00 per month, you would be entitled to receive $100.00 per month towards your down payment. A $30,000 "rent credit" would not be acceptable to any mortgage lender if you did not pay that money over and above the market rent. That $30,000 would be considered a sales concession (inducement to purchase) and is not legal.
If you are considering renegotiating your lease in order to stay in the property, get yourself a VERY GOOD mortgage banker/loan officer who can advise you on how the contract needs to be written. Make sure that person reviews the guidelines with an actual underwriter and don't take his word for it.
Good Luck