Ok Bud -
I sense you are young , and worried about the transmission because the Jeep is going to take every cent you can scrounge up.
Rust is never good , but if the frame , internally , isn't eaten up, it's going to be a lot of fun for you and your friends, and worth 5000 dollars now easily , and later too.
Have someone check it out , just so you don't waste 5000 bucks.
If you buy it , wear your seat belt , make your friends wear theirs , don't text and drive , and never drink and drive . Sorry , but I'm a dad .
Learn the TJ pledge , and never engage in any behavior unbecoming of a TJ owner , such as blocking one nostril and blowing snot or passing gas in mixed company . Be nice to Jk owners , but not too nice , just sociable and try make them feel ok. Say things like , hey , nice Jeep. They like that .
If the transmission hasn't been serviced routinely, it may he best to leave it alone . Often a complete fluid change can cause strange issues with unserviced automatic transmissions of all makes with higher mileage . Transmission fluid is so high detergent you can wash your hands with it.
That can affect internal parts that are glazed .
Anyway , thank your uncle , he's being good to you , and wants you to have it .
A first car that's a Jeep is pretty special.