Well, all the Explorer engines are pretty good. The 4.0 and 5.0 are both well proven, and the 4.6 has very few common issues as well.
The transmissions are another issue. The M5OD manual is a very very good transmission. The 4r and 5r are decent transmissions, much better than the A4LD in the early ones, but certainly not a Torqueshift or a CD4E.
If you want it for a street vehicle I'd look for a 95 to 2001. They ride nice, are decently optioned, but still have the solid rear axle which has proven significantly less problematic than the IRS setup of the 2002+. It is also worth noting that if you aren't stuck on getting a 4-door unit, the Explorer Sport kept the solid rear axle until it's demise in (I think) 2004.
If you want an off-roader, even just a weekend warrior, 91 to 94 is the better way to go. You can still swap in a 5.0 if you want to, but they have the superior TTB front end, as opposed to the independent setup of the 98 and later models.