I recently purchased a 1987 535i with a standard transmission and immediately began having problems changing gears after 2 mechanics had done inspections.

It is extremely difficult to get the car into any gear while driving or while parked. When it is in gear and I am idling and take my foot off of the brake, the car moves forward while the clutch is all the way in.

Do I need to replace the clutch and approximately how much would it cost to repair the clutch if the clutch is the problem?

the clutch slave cylinder needs replacement.
the part #21-52-1-116-300
remove the old one and before installing the replacement, connect the hydraulic line to the new unit,
make sure there is enough brake fluid in the reservoir,
then push the rod into the new slave cylinder several times until you purge the air out, then reinstall the slave
cylinder and pump the clutch pedal, it should be back to normal.
you may also wish to consider replacing the master cylinder as well. the part #21-52-1-156-000