I am not able to answer your question directly, as I am not familiar with the details of the spring ratings for the cars you are talking about. A 240D engine may be pretty close to the weight of the 280 engine so the amount of lift you get might be marginal for the effort.
I believe the most direct way to get the car up is to put shims under the springs. These shims are standard parts from MB, and if the standard ones are not thick enough, you can make some. Also, I believe MB makes a steel plate guard for the oil pan, which is probably the best solution to protect it from being punctured by hitting stuff on the road, or falling into holes and the like. If you cannot get an MB version, I think you can make one from some plate about 6 to 8 mm thick.
If you raise the car with a different spring you may avoid a few of the obstacles but you cannot expect to raise it enough to really ride over all of them. Good luck, and hope this helps, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)