What is this steering link called 85 300sd
 

Hello it seems I have a torn boot and excessive play in this adjustable link on the outside ball joint connection. Do you have to buy the whole link or can I just replace the ball with the torn boot?

That is the tie rod. You can buy just the tie rod ends. They are $12/each for Lemforder brand from this side. Or you can buy the whole assembly which is about $45 same brand.

You might find that the ends are rusted solid in the arm so a new assembly might be the best way to go.

1985 Mercedes-Benz 300SD Base Sedan - Steering System - Page 3

Replace the idler arm bushing while you are dealing with the steering bits.

X2 highly recommend this.

Thanks all I'll get on it ASAP.

Be aware that the tie rods could have been swapped so that the inner is installed where the outer was originally. This has no effect on adjustment perhaps besides the direction of rotation. Order both so you don't find that the threads don't match.