Here is what I can tell you from my experience. I had a 1989 for a while. Overall, they are very robust - but there are some common problem areas/maintenance demands:
1. The abs system - I had to repair a wheel speed sensor and module.
2. The front transfer case - make sure its operational
3. Timing belt - easy enough to do
4. Like all older gear driven odometers - the gears can break - it's a common issue, but mine never had any problems
4. Rust - not sure if this is a huge deal for you - they can rust in the rear by the battery box
5. Probably the most annoying thing was the door locks. They can seize and you will have to buy new tumblers. It is very common for only the trunk lock to operate.
6. Power steering rack - I bought a rebuilt one - it had sprung a leak.
7. Suspension - you will need new control arm bushings and related. Like any older car.
Even though I listed a bunch of things, they are very stout. The M20 is very reliable and the AWD system was amazing. I actually liked going out to drive it in the snow, and it literally felt impossible to get stuck. Mine also had heated seats which was awesome. It was the perfect car for the winter.
I would suggest that you find one with minimal rust. I really hate rust. Otherwise, a well maintained example is a very good purchase in my opinion.
Here is the obligatory picture:
