As a buyer its simple if they have the title.
- Bill of sale
- Title
- Odometer statement
- Depending on state, they might need to sign something for a temp plate, usually a bill of sale is fine
You'll have possession of the car an receipt of payment. For the seller who wants to make sure they aren't getting scammed on payment its a little more challenging.
FWIW if you're flying in I would recommend against flying with $25k cash. You are asking to get tagged by TSA and deal with civil asset forfeiture. Find a branch where they are and do the transaction there. You can get a cashier's check done in front of them and not worry about traveling with irreplicable money instruments.