Probably the transmission. As noted previously, the problematic 4matic system was on the earlier 124 chassis cars, through 1995 model year. (It may have been discontinued a year or two earlier, not sure...) The newer 4matic that debuted with the 210 seems quite reliable.

Even in decent condition, a '99 is worth what, perhaps $6K? Putting in a factory rebuilt transmission (the best, with a good warranty) is maybe a $4K repair. Given that you can't test drive it to tell what else is wrong, the car is worth zero dollars. Not sure I'd take it even for free. Even after replacing the trans to get it on the road, you might find enough other issues that a different, running car at a higher price would be a better value. Walk away.