An extended example (note I added custom arms in this example just to show it's possible, this thread was about hiding arms):

#include <sourcemod>
#include <sdkhooks>
#include <sdktools_functions>

#pragma semicolon 1

new const String:PLAYER_MODEL_DEFAULT[] = "models/player/custom_player/legacy/ctm_swat_variantd.mdl";
new const String:PLAYER_MODEL_CUSTOM[] = "path/to/custom/model.mdl";

new const String:ARMS_MODEL[] = "models/weapons/ct_arms_gign.mdl";


public OnMapStart()
{
    PrecacheModel(PLAYER_MODEL_DEFAULT);
    PrecacheModel(PLAYER_MODEL_CUSTOM);
    PrecacheModel(ARMS_MODEL);
}

public OnClientPutInServer(iClient)
{
    SDKHook(iClient, SDKHook_SpawnPost, OnSpawnPost);
}

public OnSpawnPost(iClient)
{
    if(IsClientObserver(iClient) || !IsPlayerAlive(iClient))
        return;

    SetEntityModel(iClient, PLAYER_MODEL_DEFAULT);
    SetEntPropString(iClient, Prop_Send, "m_szArmsModel", ARMS_MODEL);

    CreateTimer(0.1, Timer_CustomModel, GetClientSerial(iClient));
}

public Action:Timer_CustomModel(Handle:hTimer, any:iClientSerial)
{
    new iClient = GetClientFromSerial(iClientSerial);
    if(!iClient)
        return;

    SetEntityModel(iClient, PLAYER_MODEL_CUSTOM);
}