This is just my opinion, but other than the vehicle's maintenance history, the Ex's reliability depends heavily on the powertrain that's installed.
The overhead cam V6, from what i've read, is powerful and efficient, but has severe reliability problems with the timing components. I've also heard that the manual transmission is far more durable than the automatics, but thats anecdotal only.
The pushrod 4.0 and 5.0 are both very stout motors that will run a long time if maintained. Otherwise, the build quality of the Ex is better than most other vehicles of its time. It has some parts that were poorly made but these are relatively minor (door latches, saggy leafsprings).
I have a 2000 Explorer Sport and its reliability has been very good, most parts i've replaced as preventative maintenance, not because of failure. I've owned old and new Honda civics and accords, Chevy's, Toyotas, etc and every vehicle, even the brand new Honda civic, had problems. I will say that you should be glad you got the 4-door model because they have superior ride quality compared to the 2-door sport model (which is one of my main gripes about the vehicle). The only vehicle i've owned thats had fewer problems than the Ex is a 1993 Honda accord which must be a magical vehicle...190,000 miles and besides maintenance, one sensor has been replaced, thats IT. That's some serious engineering.
One added benefit at least here in the States is that junked Ex's are available in droves at salvage yards. This means that a lot of parts that would normally be very expensive cost next to nothing. The local Pull-a-Part here had about 20 explorers last time I was there.
