Make sure you have a sturdy test stand. By that I mean one that is solidly mounted to something rather heavy so it can't move when the engine is powered up and will not loosen up under the engine's vibration. Yes, the propeller should provide adequate cooling if it is the proper size. Most engines come with some recommendation as to what diameter and pitch prop you should use. As to glow plugs, all the major brands will be adequate. There is not that much difference in any of them. Yes, a glow plug can be very sort lived under some conditions, engine break in being one of them as there can be a lot of foreign material deposited on the plug as things wear in initially. If the platinum on the plug coil gets coated with deposits, it may not have enough reaction with the alcohol to maintain sufficient heat to keep the engine running even if it glows bright red when you attach the battery. Also, any lean run can quickly destroy a glow plug. However, once the engine is broken in and never run excessively lean, a glow plug can last a very long time. I've had some that I've been using well over a year.