Welcome to the forum, You've come to the right place.
Spend as much time as you can reading the archives--there is a lot of good information there.
RUST is the principle reason I would walk away from a car. Minor rust is acceptable and repairable; if it has gotten into the main structural members, its time to walk away. Places to look for rust include under the carpet in front of the rear seats, as well as around the jack points, and rear suspension mounts. Don't confuse surface rust with deep rot.
Vacuum----the door locks on a "S"class are run by a different vacuum system than the W123. There is a pump in the trunk that provides vacuum to run the door locks. You may hear it run--kind of a dull hum, when you lock and unlock the drivers door.
Don't be overly impressed by a pristine interior--Most Mercedes' interiors look pristine, even after 20+ years. The top of the back seat may have become dried out from the sun beating down on it, but otherwise the seats should look good. The drivers seat gets the most wear, and may even have some broken springs. This is an easy fix with a W126 ( 300SD) as a good passenger seat from a junk yard caris a perfect replacement.
A/C is typical complaint. Most lesser cars are long gone before the A/C is old enough to fail. These cars last long enough that we have the opportunity to repair this system. Look up any A/C posts by Larry Bible--he is a trustworthy authority on the subject. Often A/C problems are leaking/broken seals, which are not too difficult. If you replace the compressor, replace the reciever-drier, also. If the evaporator is shot ( thats the part behind the passenger side of the dash, its about a 20 hour proposition to remove it.
(I think thats why you see so many W123 cars with all the windows open in the summer.)
Typical areas of concern are A/C, dim dash lighting and a lack of cup holders. Thats about it for me. The more time I spend working on my S, the more impressed I am by the amount of thought that went into EVERYTHING. Its truly amazing.
Oh, one more thing, perhaps the most true of all truisms on this forum,-- There is nothing more expensive to own, than a cheap Mercedes.
In other words, if you are buying a poorly maintained car that the previous owner (PO) has neglected, you will spend a lot of money on deferred maintenance. Its better to spend more money up front on a well-maintained car than to buy a cheap one. The more maintenance records you have, the higher level of confidence that you are getting a good car.