I think you are not allowed to change player weapons skins anymore because valve claim it will damage their economy and will ban your server from the community list. The irony is that, in court, they claim their skins has no value to defend their "gambling" business (case opening).
For player models you can use SetEntityModel from sdktools
You will need to precache the files OnMapStart and add the files to.
This question was asked allot on the forum and therefore not really appealing to respond, also there is tons of plugins that do that and can look up the source code.
https://www.sourcemod.net/plugins.ph...tion=&search=1
https://www.sourcemod.net/plugins.ph...tion=&search=1
// to set arm model
SetEntPropString(client, Prop_Send, "m_szArmsModel", "<arm_model>");
https://sm.alliedmods.net/new-api/ha.../PrecacheModel
https://sm.alliedmods.net/new-api/sd...SetEntityModel
https://sm.alliedmods.net/new-api/sd...DownloadsTable