George,
As a proud owner of a S320, ABS problems are always a wild goose chase, and can have very high dollar ending. The problem could be one of the wheel sensors too. Take a look at this thread:
ETS/ABS lights in dash 1995 S320 - Benzworld.org - Mercedes-Benz Discussion Forum
With the ABS problem looming around. I hope you are getting the car for under $3000 maybe even $2000.
A big weakness in with the W140 is reverse gear, and the problem is a 10 dollar o-ring inside the transmission. Which happens around 120k to 150k. The sign is when the transmission makes this roller coaster clicking sound. Mine started to make it around 120k and failed about 135k completely.
My mistake in owning a S320 W140. I should have purchased one of years of 97,98 & 99. These came with is more reliable 722.6 transmission. Currently their is a thread going on "722.5 teardown" here on this site. I have my transmission in a million pieces.
I do not know if you are a DIY'er or not. I would not purchase a running vehicle from any dealer with a instrument light especially a ABS.
If you do find your, dream W140 car. Make sure you change the heater plastic tee as this tee is good up to 80k miles, and any time after that you are on borrowed time. When the tee breaks you have about 30seconds to 1minute to react before you ruin your engine permanently.
the evil that lurks under the hood... is really the devil himself - Benzworld.org - Mercedes-Benz Discussion Forum
Not trying to scare you. I'm want to find a 97 and above S320 as like the lines of W140. It is a timeless look.. Having a W140 has its flea's and killing them is important, or they kill you...
Best of luck,
Martin