A single MP40W in a 120 regardless of what type of coral you keep is definitely not overkill. You'll find the far end of the tank will need supplemental flow and will most likely want to keep running another powerhead on that side. I run two MP40s on my 4 foot 77 gallon. They're dialed down a bit which is nice, since they run a little quieter at less than full speed.
I have a feeling you will be very impressed once you see one working in your system, and it won't be long before you'll be thinking of adding a second. Your options increase by a large magnitude with two, since they are able to communicate with each other in Master/Slave mode.
My first pump is almost two years old now and was Gen 1 with old magnet. I replaced that after about 6 months with a low cost kit from EcoTech. Second pump was added about a year later. It was a Gen 2 and I was finally able to upgrade the firmware of the first pump to get it to final Gen 2 status as well. Both have been running without fault since I put them in service. About the only possible issue I can foresee is that you may find them somewhat noisy since the motor is outside the tank. It seems to vary from one to the next. I don't find either of mine a problem in that regard, especially since I run them at less than full power. But each one sounds a little different, the newer one maybe a tad more quiet. If you use short pulse mode with the pump ramping from idle to 100%, you will hear that in the next room, depending on what other noise you might have going on.
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