I've had a 93 400e for the last 2 years and it has been excellent so far except for the oil leak which I haven't diagnosed yet. The things to look for are:
1. Wiring harnesses - make sure they're in good shape.
2. Power steering pump - it is notorious for leaks.
3. Timing chain - make sure there is no noise or other problems with it.
4. Service records.

Since this car has only 15000 miles other than the wiring harness I wouldn't expect to see any major issues with it.
As for the ASR, I don't know if it can be turned off. One thing I can tell you is that once you drive this car you will NEVER want go back to a 6 cylinder. It is smooth & powerful and addictive.
Hope this helps.

smk_texas
93 400e 126k
99 Mercury Villager 70k