I have the same car with 196,000 miles on it. I have had it for 8 years. By far the best car I have owned. I use synthetic but the motor is bullet-proof so it may not matter too much. Common problems:

Wiring harness: mine was in great shape so I have not had a problem but they are known to fail (especially after doing a head gasket job).

Head Gasket: just had mine done 7,000 miles ago. Most begin leaking after 120,000 or so. Look for motor oil in the coolant.

Transmission: just started having problems with reverse gear. This is common as you get closer to 200,000 miles. Make sure when you have the trany serviced they drop the pan and do the filter too, (not just pull the plug and drain).

Engine Mounts: replaced 30,000 miles ago.

Flex Disk: replaced 60,000 miles ago.

SLS: never had a problem.

AC: lost it a long time ago, but living in northern New England I don't need it.

Good luck with your new car. They are fairly rare, in fact I have yet to see another 1995 E320 wagon in person!