Your description of the Trek 800 seems bang-on. The 800 was Trek's entry-level mountain bike. It has been replaced with the 820, which retails today for $349.99 USD, so $240 in 1997 dollars sounds reasonable. That being said, $100 may be a little steep for a 14-year-old bike, but if it's in the shape that the owner claims then it might be worth it. The "Tourney"-level components on the bike aren't Shimano's best, but they've been around for years and are still being produced. A well-maintained Tourney drivetrain should give you years of faithful service.

You are correct in assuming that 50 mm of travel refers to a suspension fork. In entry-level bikes suspension is cheaply made, heavy, and unneccessary... especially if you're just using the bike for commuting. If you want to tackle some singletrack then be prepared to spend some real money and get a MTB with quality suspension.