I don't believe the wagon ever came without the self-leveling system. I have a full line broucher for Mercedes, for model year 1980. It mentions the leveling system as being standard. Mercedes introduced the TD in 1978, so perhaps if it ever did have a "regular" suspension, it would be in the years '78 and '79.
I don't know that you can just remove the hydropneumatic shocks and put in standard shocks. From what I have read here, the coil springs in the wagons are not strong enough or meant, to hold up the backend. They are only designed to suppliment the leveling sys. I guess you could try and put in the springs that are for the W123 sedan, but the wagon weights more. The common thing to go bad are the nitrogen cells. The cost of these is alot cheaper than new shocks and new springs.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section