Not wanting to presume anything
This car you speak of, what year is it? Your problem seems all to familar to me when I first got my Benz. Slow cranking, then no cranking and definitely no start. Has this car been sitting around undriven for any amount of time? I'd do the following now, if this was my car. First, charge the battery, then take it to either a garage or Wal-Mart and have them put it on their tester to check for shorts or a dead cell, they do it free of charge. Your battery you pulled out of your car may only have a cranking amp of 550 or so and most diesels really need 750 and above to do the preglow and cranking necessary for ignition, mines a 850 cranking amp and 930 CCA. When you find out the battery is good and it's the proper amperage then try it again. I've been through the new battery that won't turn over a Benz diesel already. Jumping one of them is next to impossible as well unless the battery in the Benz hasn't really lost it's charge. The next item I would then look at is all the connections from battery to starter for perhaps a bad connection. Then when all else has been eliminated I'd consider the starter as one that perhaps has bad brushes or bearing causing a substancial draw on the battery or drag that is chewing up the amperage sent to it causing it to turn over extremely slow. If the car has been sitting a long while or cranked over quite a few times without success, I'd inject a little motor oil into the intake so you can lubricate the upper head and rings for a easier engine turn over and increased compression. After all has failed then either replace the brushes in the starter or spend the $150 for a rebuilt Bosch and go from there. I've done the whole senerial and ended up with a new battery that was underpowered and a rebuilt starter that was put in by someone else, paid too much as well. I've learned a lot since then and doing things in order saves money in the long run.