Jonathan, welcome to the forum.
If you do a search for Mugnaini, you'll see that the reviews here in the forum are mixed- at least from a Neapolitan pizza perspective. I'm sure that a Mugnaini will have no problem with a coal style pie (such as Grimaldi's), but, you never know where your tastes will go, and it would be a shame to spend so much money on an oven and not have it be ideal for Neapolitan.
Right now, I would say the Four Grand Mere is looking promising. I'd like to see a few more members taking the plunge before I fully endorse it, but, compared to the other pre-fabs out there, I don't think anything comes close.
Btw, it can get tricky depending on what kind of oven you have (gas vs. electric), where your broiler is located (main compartment vs. separate broiler drawer) and the oven's peak temp (550=easy, 500=harder), but it's not that hard to get a Grimaldi's pie out of a home oven, as long as you have the right stone/setup. By all means, strive towards your outdoor kitchen paradise, but, in the mean time, you might as well see what you can get out of your regular oven.