Basic system, do with it as you will

local myModelsTable={}
myModelsTable["STEAM_ID_HERE!!!"]="models/player/kleiner.mdl"

hook.Add("PlayerSpawn", "checkForMyModel", function(ply)
    if myModelsTable[tostring(ply:SteamID())] then
        ply:SetModel(myModelsTable[tostring(ply:SteamID())])
    end
end