@Stef914 Welcome aboard. This is a great place to learn more than you will ever want to about the 914. No shortage of information and help. The attached video of Dr. 914 doing a good PPI (pre-purchase inspection) is well worth the time.
As others have said, rust is the biggest concern on these cars. Next comes accident damage that was not repaired correctly. Remember these were inexpensive cars and often times corners were cut. Really only in the last 10 years have these values steeply climbed.
Take the time to educate yourself on what to look for and what to run from. Then be prepared to act when the right one comes up so you don't miss it. Typically the better condition the car to begin with the better off you are in the long run. I would also add, the closer to stock you find the car generally the better off you are. Also Ian Karr's videos series is well worth the time when it comes to seeing how to work on these cars. Very educational and entertaining.
Good luck in your search and enjoy.
Michael