Trying to structure a deal with my parents. HELP

[Chris Breezy]

So my parents inherited a co-op in Brooklyn ny. Where me and my gf are gonna move in once rehabbed.

current value- 315k fully paid off 

Rehab- gonna cost me 20k since I’m doing it all myself 

Post rehab value - 345-350k

1. 
   the hoa that I will be paying is about 600 a month. My idea is to get is assessed to a exact number and get a written document stating that and then down the line once sold I make the difference in appreciation since I’m laying out the 20k rehab. Is this beneficial to me? Any other strategy’s that will better benefit me?