Why not just ohm out the plugs from the valve cover gaskets once a year, then you are done and don't have to purchase anything. Also since the glow plugs resistance will decrease as they warm up you may not get a accurate reading all the time.
I'm with Bugman. Glow plugs don't go bad that quickly. Just check the ohms thru the VC gasket connector every 6 months or so if you're concerned about it. Or maybe just in the fall before they're really needed.
I've put a clamp-on ammeter on the GPs and they draw over 100A if that helps you select a CT.