There are a lot of things you need to look for when you are buying a classic car, such as:
Check the body tag
Check that the interior parts are original
Pat down vinyl roofs, look for spongy spots
If the vehicle does not run/drive, check the last time the car was driven
check the local market value and how much it will cost to restore
The more original the better, so check that major components have not been swapped out.
