FWIW, that's a later-production ZF out of a Pantera. It was probably made in the 1980s sometime as evidenced by the ID tag on the right side of the case, rather than on the left side (shifter box cover).
Any used ZF like this is a crapshoot. I recently bought a long-sitting gearbox like this, which looked to be in similar condition. Once we opened it up, it was a bit of a disaster inside. All the gears, ring and pinion etc. were in great shape, but there was evidence that it had been dropped while in storage and a surprising amount ($$$) of internal damage took place. Fixing it was a straightforward process involving the replacement of several key components, none of which was cheap.
How well do you know the seller? Would he entertain an offer of X dollars/pounds minus the cost to repair any hidden damage? That would require opening it up for inspection and delivering it to the hands of an expert well-versed in such things, capable of both repairing any damage and making a cash assessment of the cost to perform such repairs. That's the deal I had with the seller of my gearbox. I made an initial offer, pending a successful internal inspection; once it was opened up, my offer to him went down commensurate with the additional cost to repair the damage (above and beyond the expected costs of replacing 2nd gear synchro, etc. which are part and parcel of a rebuild of just about any ZF)
Although he was disappointed that he didn't get as much for his gearbox as he hoped, he fully understood the reason behind it and there were no hard feelings.
