You're going to be pretty underwhelmed by the Intel graphics. Yes, it will run... No, it will not look great. At the minimum, I suggest looking at an iMac with a dedicated graphics card. Unfortunately, this isn't available on *any* of the 21.5-inch models in the current generation of iMacs---you have to look at the 27 inch models, which come equipped with graphics cards ranging from the Radeon R9 M380 up to the Radeon R9 M395X.
For what it's worth, I work on a late 2013 iMac with Nvidia GeForce GTX 780M graphics, and X-Plane looks pretty good (but, unfortunately, not as good as if I were using a Windows machine that cost a comparable amount---simply because the iMacs have used the *laptop* versions of graphics cards for a long time now, rather than the much more powerful *desktop* versions of graphics cards).
