I may come down to personal preference so it's hard to say. Based on certain standards it may technically be underpowered for 120" but you may be fine with it.
I only say this because I've been using a unit that barely qualifies as a projector, the 50 lumen LED Optoma PT100, and I've been enjoying it on a piece of white painted foamboard for the last 2 years in my office. It's only about 63" and according to ProjectorCentral calculator, I'm seeing about 2 lumens worth of brightness!!
But with the lights out, I've been pretty happy with it (for what it is and what it cost). This being my first projector also helps no doubt lol...
I've also seen threads where folks were projecting 90-100" images with mere 100-200 lumen LED projectors and were also happy with it. So it may come down to what makes you happy and perhaps what screen you use.
Just my few inexperienced thoughts:p