You gotta write a path for every weapon model, and a new name (v_shitm4.mdl instead of v_m4a1.mdl).
You can do this in amxmodx, I forgot how to do it tho.
You need to place all the weapons you want in your dedicated server folder, rename all mdl's. Try to keep the polies and textures low and skip w and p models for faster download time.
After you have placed all the weapons you want to replace, you need to create a simple plugin, now I forgot everything about amxmodx coding so I don't know how.
But I know it's possible, I have done it.
You need to make it in the plugin so people joining the server download the models and define that v_shitm4.mdl for example replaces the m4a1.
This is't that advanced really, just takes some time to write all weapon paths in the plugin and get everything right.
I remember this server, well I have seen several, but it was one server I really liked.
It was called "Baracura" or something like that, and it was a new weapon model for every model.
It was't just downloaded models from fpsbanana, but every skin was edited for a special look.
I can try to find the old plugin I made, if I still have it.
You might find something at amxmodx.org.
In order to get skins on server which everyone can see, use plugins.
i use this for player skins