I'm not an expert on the W115 240D, but I can give you some basic answers:

1. The original oil cap was metal, but the replacements look like this:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ425G02GFSB&year=1976&make=MB&model=240-D-001&category=All&part=Oil+Filler+Cap

You can also replace just the gasket:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ425G02GFSB&year=1976&make=MB&model=240-D-001&category=All&part=Oil+Filler+Cap+Gasket

I find that putting a little grease under the cap also helps seal it.

2. Glow plugs are simply electric heating elements that pre-heat the air in the cylinders so the engine will start on compression. They do burn out, sorta like light bulbs.

3. Your car has glow plugs wired in series such that if one burns out, none will work, which is probably why the light is not coming on and the car is hard to start. The glow plugs look like this:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ425G02GFSB&year=1976&make=MB&model=240-D-001&category=All&part=Glow+Plug

You should be able to identify the failed plug(s) with a volt meter. If you search, there is a bunch of info here on trouble shooting glow plugs.

4. Regarding maintenance; Change oil/filter (normally 3000 miles for conventional oil or 5000 miles for synthetic - use a diesel rated oil), fuel filters (There are 2) that should be changed if you notice a lose of performance:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ425G02GFSB&year=1976&make=MB&model=240-D-001&category=All&part=Fuel+Filter

Belts and hoses (just like any other car), auto transmission fluid/filter (every 30K miles), adjust valves (every 15K miles), coolant (every 2 years with MB coolant or Zerex Z05), I'm sure I'm forgetting some.

5. The AC noise is not normal, start with the belt and see if you can locate the source of the noise (hopefully not the compressor).

6. These guys sell carpet sets, but you may find better prices elsewhere:

http://www.adsitco.com/category.asp?CID=1558&VID=

7. The Haynes manuals are "OK" for minor stuff and wiring diagrams, but no substitute for the actual factory manuals. It's probably worth picking up a Haynes (I have one).

This is the best place to post diesel related questions.

Hope that helps.