Private Lender for Buy and Holds

[Joe Threats]

Ok so there's a property that's been rehabbed and on the market for 45k it was originally listed at 59k. 

I have someone thats wanting to move out of there current situation that is will to pay 800-1000 for per month for that exact property. I told them depending on what I get it for that could possibly happen. 

I plan on asking my parents to pay cash for this property so i could offer a lower price a close quicker.  What percentage should I propose to my parents? 

Thanks in advance!