The long standing story is that when GNR got signed, Slash had a small collection of guitars- Jacksons, Strats, BC Rich, that just didn't sound good to the producer. The producer brought in the "Max" Les Paul since it was a more accurate and better sounding Les Paul than Gibson was making at the time. The builder had a supply of maple, mahogany and brazilian rosewood that was leftover from the 50's, so building it with this wood essentially gave you a 50's Les Paul. That's why they cost so much. There was another replica that was brought in Chrs Derrig that was subsequently returned, and no one knows if it was the MAX or Chris Derrig Les Paul used for Appetite for Destruction.
Same goes for the Marshall amp used- there's a bunch of contradictory stories floating around as to what happened to the Tim Caswell modded Marshall