BrickUnderground

I'm in contract to purchase a co-op in Brooklyn.  I love the apartment, but the owner occupancy rate of the building is ~20% (sponsor still owns half the units and other owners in the building are renting) and so far, no bank will give me financing.  The building recently implemented a 2 year cap on renting and the sponsor is trying to sell one of his units so the owner occupancy rate should go up in the next 2 years, but it's unlikely the owner occupancy rate will go higher than 50%.  The sellers are trying to get a Fannie Mae waiver (I'm not really sure what that means) and are also pursuing the bank who has the underlying mortgage on the building to see if they will provide me with financing. 

Like I said, I love the building, and negotiated a pretty good price, but I'm nervous about having major problems when the time comes for me to sell (in ~5 years or so, maybe longer).  Has anyone been through this before and can give some advice?