The owner (Ebay ad) says the boat was left in the sun? Sounds a bit implausible. These boats were sold around the world including hot countries such as Spain, France, Greece. More likely that there's been a fuel leak which has delaminated the floor adhesive. That bulge won't neccessarily cause problems, but it's likely to get worse.
Decent Zodiacs come on the market now and again. Just a case of waiting for the right one to come up. Don't rate Selva motors, but ok as an anchor!