The most common cause of coolant (should be a 50/50 mix of coolant & distilled water) coming out out of the overflow, located above the right rear wheel, is a bad coolant cap, or an air bubble in the system.
If the seller says he was adding water, it is probably an air bubble that is causing coolant to come out of the overflow. I would ask him if he was using distilled water, if he was using regular tap water you should drain, flush, and replace all of the coolant to avoid any minerals building up in the coolant system. This would probably cost a few hundred dollars/euros to have someone do.
If he was using distilled water, you can open the purge valve under the plastic cover (
see this link) and drive the car for 2 days with the valve open to let any air in the system purge itself.
Check the rear truck, around the tank/under the carpet to make sure there is no obvious leak. Look for dried white powder, the OEM coolant has an additive that dries white so you can spot any leaks. If the tank is leaking it will need to be replaced which can be pricey. The tank alone is about $400 USD.
I would also recommend getting a Pre-Purchase Inspection from someone familiar with Porsches, repairs for these cars are not cheap.
Best of luck!
