The client does not hold quests, npcs, items or anything of that nature. It contains the game itself you use to play, the User Interface, the models of every race in the game (but not specific npc data itself), and the zone models. as well as a few other things. Everything else is handled by the server. Quests are run through perl scripts which you can find in the "Quests" folder for the server you create. NPCs, NPC path grids, pets, Items, mounts, player data, server coded rule sets (xp rates, max level, expansion variable etc), one half of the spells (spell data and what reaction a spell creates is held in the database. Then there's a spell file in .txt form in the client that, similar to the string files, provides the visible data used to see and use spells in-game.), and other customizable things of that nature are held in the database.
You can change all the quests or npcs you want and the zone will still physically look the exact same. If that is your intention you don't need to know anything about the client files and how to fiddle with them. On the other hand, were you to completely change the physical appearance of a zone through the client files, the npcs, items etc will still remain the exact same (though you're likely to run into mobs spawning at locations on the grid that place them above or below the world, causing them to fall in endless loops and glitch around a lot until you change them). If this is your intention that's beyond basic server development. You're going to need more than S3Dspy. S3DSpy only allows you to view the contents of an s3d or eqg to extract the textures. The textures are the image files laid overtop of the mesh to create it's appearance. The mesh is held in the same s3d or eqg files as a .wld or .mod file (for s3d and eqg respectively) and form the 3d skeleton that the textures lay onto. There is a few tools that can be used to mess with the mesh but they are mostly unsupported anymore and they were never far enough along to allow a complete rebuild of an npc as far as I know. Zones, on the other hand, can be rebuilt from scratch with OpenZone. But you can't modify existing zones that way. You can find the OpenZone sub-forum near the bottom of the main page with downloads and guides.
If you're looking just to edit the textures I
wrote this awhile back. The info is slightly dated but it's enough to get the job done.