1) Compression pressure on a Diesel is very high (compression ignition). If you want a Turbo model, the normal reading is 24 to 30 bar (1 bar equals about 15psi), Min reading is 15 bar, and max pressure difference between highest and lowest reading is 3 bar.
Readings for a naturally aspirated model is the same, EXCEPT normal reading is 22 to 24 bar. These are the readings you should get on a warmed up engine. It's a liitle more work to do a compression test on a diesel, normally I would check in the injector hole, but you need to replace injector seals if you do, You can also check through the glow plug hole, but it's a shame to pull the glow plugs without replacing them.
2) Use this site (Fastlane) or Benzbin I hear is another. If you hang out long enough you'll hear of different resources for parts. MB parts are generally more expensive than Ford parts, OK? And not alot of "custom" parts either. You won't generally find resources for a chrome differential cover for a 300D, it's usually not considered "that" kind of car. To find performance parts for a MB usually takes alot of hunting and cash.
3) You can still get a jerk when shifting with one of these. A site like this can help you figure it out, although it IS true MB's shift a little "firmer" than many other makes. A firmer shift means less slippage when shifting which means less transmission wear. "However" there is kind of a common problem on these diesel trannies. They use essentially the same tranny as a MB gas engine car, and do a little vacuum magic to "fool" the transmission into working the same as if it were in a gas engine vehicle. The engine has a vacuum pump, mainly to run the power brake booster, but also as you'll eventually find out if you buy one, to run things like the power door locks and, like most other cars of this era, to run the climate control. Well, on a gas engine car, the engine vacuum is also used to run a part on the tranny called the vacuum modulator, which controls how hard or soft the transmission shifts according to how hard the engine is being accelerated. Low vacuum means the engine is being accelerated hard, which means the transmission should be shifted hard to avoid slippage. High vacuum means the engine is being accelerated very slowly (high economy), which will give a soft, easy shift, more comfortable feeling.
What can happen is that you can either A)develop vacuum leaks, B)heve some misadjustment done, C) have a worn vacuum pump, or D)get a restriction in the vacuum line which supplies vacuum to the transmission (the brake booster line), any one of which can lead to low or no vacuum to the vacuum modulator, which more or less makes the tranny "think" the engine is being accelerated hard, and will give, as they would say on a football field, "unneccesary roughness". (Boy, long ways to go for such a lousy punchline). But any of these could be what's going on. It's not the end of the world, it certainly is repairable in competent hands, I've seen this alot.
4) No (hey, a short answer for once!)

Gilly

PS You can't use a compression gauge for a gas engine on a Diesel, you'll ruin it, the pressure is too high!