Steering Gearbox slop?

[NWFMB]

85 300SD - how much slop in the gearbox is too much? Is there a good source for used parts, I noticed that I new one is about $600-$700. Also when I had BigO tires rotate and balance the tires, they noted that there was excessive play in the tie rod ends, I got under it while someone was turning the wheel was not not able to notice any play in the steering linkage at all, only in the box, I tried to tighten down the allen bolt, but did not make a big difference.

Any advise would be helpful, I normally do most of the work on my own vehicles, but have never owned a MB before (just bought it last weekend)and my only other experience with diesels is our Dodge 3500 Cummins.

Thanks