Big thing when buying a first gen is the timing belt, needs to be done every 100k. If a shop did it, then there should be a metal tag near the center of the radiator. Other thing is assume plug #6 has never been changed; it's in an awkward location that many people have issue getting it freed from. Make sure you run it though all the gears, let it idle after it's warm to check the cooling system, make sure the 4x4 engages (if it has one), the usual things to check when buying used.
Remember: this is a box on wheels with a minivan engine. Expect it to get from point a to point b, but not very fast.
Once you have it, flush all the fluids (never trust the previous owner) and keep up on the regular maintenance. It's a Nissan, so it'll run for a really long time.