That's what the set_task() is for... add a repeatitive task ("b" flag, you're using it already) at the start of plugin, check your stuff and then send the hud message to everybody.
You also might want to extend the HUD message duration and/or reduce the task delay... a 1.0 for both should be enough for counting down seconds.
That client_PreThink is verry WRONG here, it's called dozen of times per second for each player... and you're sending a message to all players from each player's prethink... imagine how much useless traffic you're adding.