These cars are very reliable. I've owned, bought, sold and fixed many mercedes. The maintenance costs on them is comparatively less than their gas counterparts. If you do regular preventative maintenance, and make sure your fluids are good it should last you for years to come.
General problems with them, as with most Mercedes are:
1.) Vacuum System problems (almost everything from the transmission - to door locks are vacuum controlled... Its possible for the lines to get old and crack and develop a vacuum leak.) I've had a couple mercedes that it felt like the transmission was on the way out, but it turned out to be a simple split in a vacuum line. Common problem, but not too terrible expensive if you have a competent mechanic that can do a vacuum check on the lines
2.) Window Regulators fail... Thus window stops going up and down.
3.) Sunroof Motors go out.
4.) Window switches burn out.
Those problems are inherent of just about any mercedes, even up to the later model ones. They are minor problems.
Now as far as reliability, I've seen these cars push almost 400,000 miles, with the potential for more. The insurance on the cars is very cheap, it's an older car, enough said. As far as gas mileage, it would make a great commuter car.
Now as far as your ad, I've seen the same ad. $9000 is WAY to steep. Everyone is right, expect around 3,000-5,000 maybe less if its pretty thrashed.
If you live in the Bay area. I have one for sale at the moment. I'm askign $3200 obo. My cell is 408 - 561 - 0855 if you want to come by and take a look. Runs well, interior and body are very clean. Definitely a lot cheaper than 9 grand, and in comparative shape. Only a few minor things that should be addressed, new drive belts, tires a little old, and brakes are getting old. It does however have a new waterpump on it!
My Cars For Sale/Sold Call 408-561-0855:
1975 Mercedes 450SLC (SOLD $1800)
1976 Mercedes 300D (FOR SALE $3200
1977 Mercedes 300D (SOLD $1300)
1979 Mercedes 450SLC (SOLD $2200)
1979 Mercedes 300SD Turbo (SOLD $3100)
1984 Mercedes 240D (SOLD $1600)
1986 Mercedes 560SEL (SOLD $2800)
1986 Mercedes 560SEL (SOLD $2500)
1986 Mercedes 560SL (SOLD $5900)
1989 Mercedes 420SEL (SOLD $3200)