What you mean by "use the car" ? Once grey card is canceled, the owner would take license plates with him/her. So yes, you can sit in your new car, but that's about it.
If you want to move the car to your parking, take out temporary 1 day license plate if your canton provides have such option. Alternatively discuss with the seller, maybe you can pay for train ticket and he/she can drop of the car at your place.
Maybe the best way is to take the morning to get the gray paper with the seller, go get the plates and register, and return the plates to the seller.
The car will be deposited in my place since today until he receives the payment on Monday. So after he receives in his account, he will invalidate the document.
Flooding isnt the same as flooding... thats seawater damage in cars full of electronice. Given the costs to repair anything in Switzerland... forget about it for here. But maybe get it to Mexico to fix?