1. If you plan to do your own oil changes, a device that uses vacuum to extract the oil through the dipstick tube is a popular but not universally accepted method. You can use the same device to extract excess transmission fluid. You might check an auto parts store for a generic fluid suction device with a long tube that fits in the transmission dipstick tube. The transmission pan has a drain plug so you can let out some fluid that way without needing a new pan gasket.
2. I've never heard of synthetic gear oil causing problems with differential seals.
3. On other MBs, you need special tools that aren't economical for the DIYer to own. It's best to take the control arms to a shop and have the balljoints replaced there. Te W124 might be a different beast in this regard.
4. The first place you should check is FastLane using the button at the top of the page. I don't like depriving the forum hosts of revenue, but you can check the parts forum for factory W124 CD manuals. You can also check ebay, but most of the CDs on ebay are unauthorized copies. A UK Haynes manual is available from UK on-line bookstores and several US distributors. It's not a factory manual but seems to serve me better than most folks who post their opinion of it. There's another hardcopy manual for the W124 that you can find more about though an archive search. There's also an Alldata subscription service if you have a good internet connection.