Questions on Automatic trans!!!

remove the trans pan and check for metal chips. you may find something that helps you deside what to do.
BTW yes it could be that the linkage is out of adjustment and the modulator could fail and start sucking oil at the same time?

Inspect, inspect, inspect.

Oscar,

These transmissions are tough!! If the car simply won't shift into park I would bet the seller's presumption that their is a fault with the shifter linkage is accurate. Also, there may be something simply disrupting the shifter's travel. You can easily inspect this right from the driver seat. Remove the shifter t handle, boot, and the bezel. Look inside and see if if there is any interference there.

As for the 4th gear shifting issue; I am betting that the transmission is simply low of fluid. My 84 auto had the same symptoms when I had a cooler hose leaking on it at the radiator. To check the fluid, you will have to jack the rear of the car. The resevoir is in front of the crossmember on the passenger side of the car. Sometimes it is coated in grease and can be difficult to identify. Run the car up to operating temperature and leave it in neutral when checking the level of course.

Sort these issues before committing.