1. Owner's manual is missing. (Suggested sources for a manual?)

Try the dealer or ebay. Check the dealer first to make sure you don't pay too much for one on ebay.

2. Cold start-up. (MAF, coolant sensor, problems?)

Could be any number of things. Probably not the MAF if the only problem is hard starting. Probably best to start by checking residual fuel pressure over time with the engine off. Probably even better to use your symptoms as search criteria in this forum.

3. Radio face plate is missing. (Replacement possible or rip it out and start over?)

Assuming it's a Becker Gran Prix, contact Becker USA (search for Becker's contact info on this site). You can find replacement units in the classifieds section of this forum or ebay but you'll need the VIN of the car it came from so you can get the ant-theft code from the dealer. Probably cheaper and more rewarding to get an after market stereo unit... even if you use cassettes.

4. Technical manual. (Suggested sources?)

MB publishes shop manuals on CD. Check the FastLane and PartsShop links above.

Sixto
91 300SE
81 300SD