The "T" in TD stands for "touring." The 300TD's first received turbo-charging in 1981. All 123 wagons had hydro-pneumatic leveling in the rear only, and used a linkage connected to the rear "anti-sway" bar to sense when the rear was loaded or un-loaded and compensate by raising or lowering the rear of the car by means of a hydraulic pump adding fluid to nitrogen filled spheres connected to the rear struts or by the release of the fluid through a valve.
The system is pretty durable, and will possibly change ie;leak down, after periods of very cold for long durations. It should immediately raise to "normal" height within minutes of starting the engine. The front should have a pronounced gap between the top of the tire and the fender.
There is an additive which you can pick up at parts stores to keep the fuel from "gelling" which is actually caused by the precipitation of wax crystals due, caused by paraffin which is a natural element of diesel fuel.
Keep in mind that there is no more expensive item than a "cheap" mercedes. these are precision crafted automobiles which require diligent "care and feeding" and if this vehicle appears not to have had that, I'd pass. The fact that it was buried on a dealers lot does not bode well as evidence of this type of care.
Check Ebay or the Autotrader for examples of other cars available, and by all means, have a pre-purchase inspection performed by a mechanic who is familiar with and experienced in the repair of the model which you are contemplating.
Check this forum or try a search on Mechanics or shops in your area.