Your questions are hard to answer because it depends a lot on the individual owner and how he/she feels about their car. It depends on how much "slop" you're willing to tolerate on your car. You can do nothing and drive it into the ground. Or you can wait a bit and have the engine overhauled. A lot depends on the condition of the body.
Steering should be repaired on an as-needed basis. Look for excessively worn components and replace them.
Barring leaks, I believe the industry standard for oil consumption is a quart every 500 miles before something MUST be done to bring the engine back into roadworthy condition. It might be that beyond that rate the converter and plugs are adversely affected.
95 E320 Cabriolet, 131K