My son bought 2 JD 4020s from a farmer that was down sizing last spring for a packaged price of $13,000. I sold one before we even picked them up later that day. His daddy had bought them new in 1970 and 1971. Both had a o ring leaking on engine cooler. It took about 15 minutes to fix. The one we kept I had to install a water pump kit, replace the PTO control seal under dash and tape up a naked wire.
Total price on oil $145.00 and kit and o rings around $160.00.
Except for a o ring or gasket or hydraulic hose I expect it to last longer then I have left on this earth.