you can actually use up to the underfoot client pack, (2 expansions ago?). but some of the zones (4) are funky and someone will have to send the old ones to you.
usually, players need Toxxulia forest aka mistwoods (all the zone files ) Cyrtho Malath aka ss something (need all the zone files) i want to say Nadox, and 1 other.
Don't forget the files for Thazeran's Tower and ... I'm sure there's another I'm missing...
Also, there's no npc graphics for Blackburrow and a number of other encounters including Blazewind (help add to this list!)
That all said, I use the underfoot compilation myself and it's been fine outside of those zonefiles, and the "just shows up as a human" npc graphics haven't been gamebreaking.