Silver Spear,
Well, it isn't as easy as you would like it to be. I've seen this before, and as a former President of the local MBCA (that's an acronym you'll see tossed around here a bit; it is the Mercedes-Benz Club of America) I've received many phone calls about it too. People with almost no knowledge of one specific model (300SEL, 280SE 3.5 Cabrio, etc.) and then they want to learn "everything" about the car in order to make informed purchase and restoration decisions...in about 2 weeks time!
There's no easy way of condensing years of experience and ownership into a quick set of "sticky" notes. I think many of us would freely admit we learn all the time which is why we keep coming back here, to learn and to share what we have learned.
The Technical Manual might be a good place to start but might, in some parts, be far too detailed and specific for you at this point. Something you can do from Beirut presumably, is start to buy a bunch of books on eBay or Amazon or from the many auto booksellers in Europe.
Answering some of your questions:
"The market" places a high value in terms of price on the 280SL. You cannot assume from that, however, that the 280 is "the best engine", or the 280 is the best car either. There are quite a few 230 and 250 owners here that have run their cars for years if not decades and wouldn't trade for a 280. Yes its bigger, but not more power. Yes it has more main bearings, but the 230 engine is hardly delicate.
There are all kinds of ways to investigate the body, be sure you know how to tell original if that is what you are after. Ditto for "complete".
Check the suppliers list for parts, and shop around. Here in the USA, K&K is known for sheet metal parts. So are local MB dealers and the respective classic centers in Fellsbach and Irvine, CA.
Some parts are very expensive, such as the front grill star; the grill suround. That's part of searching for completeness. If a car looks like it is of interest to you, but is missing something, better be darn sure you can find the missing parts before you make an offer. Some trim parts, internal trim parts, and other minutiae can be hard to find.
Soft top missing: beware if the frame is broken or missing. That is very hard to repair or replace. The actual top itself is a $500 or so item, maybe more depending on brand, and then the installation on top of that.
Regarding the automatic, best to look for one that works well and is adjusted properly? If the car shifts smoothly, runs well, it is probably in good shape. There are a few firms out there that will rebuild them, too.
It is always less costly in the long run to buy the best car you can, rather than one missing parts or needing something. Of course in Beirut, you might not have a great opportunity without travelling to Europe or USA to find suitable candidates.
