And what exactly would the sales price be with the 17-18k lump sum? As far as I'm concerned, that's money straight off the purchase price. Also, partial interest = reverse amoritization = leverage working against you. Ok for very short term deals, deadly for anything else.
If, 165k sales price and 17-18k cash brought to the table and you pick it up subject 2 the current loan and the buyer is willing to help pay the mortgage for a bit (if this is what partial interest means), then you have a passable deal.
165 - 18k = 148k / 186k = 80% LTV - the starting point of a decent deal, but absolutely no room for wholesaling profit. Help on payments and the loan in the sellers name makes the place cashflow break-even so there's the possibility of future rent appreciation and loan paydown for a buy and hold.
The big problem with this is, the deal is not at all clean. You are leaning very heavily on the current owner and if anything goes wrong, it would look bad in the eyes of outsiders (you 'stole' the property with a QC deed, made the person pay you a 10% fee, then forced him to pay partial rent while you collected all the profits? Guilty. Next case.) Once the seller is out from under the property and his assets are out of the fire, he won't be willing to keep supporting you.