Logistically, you do not need to get a top-tier CPU if that is out of budget. But if you would like X-Plane 11 to be adequately run on higher settings, I would certainly recommend an i5 or i7 CPU (preferably of the more recent generations) capable of at least 3.5GHz.
Currently, I am using an Intel i7 4790k (4th Generation) with a base clock of 4.0GHz (overclockable to 4.4GHz), and it performs quite nicely with X-Plane. But please do note that equally, if not more important than selection a CPU is selecting a quality GPU (graphics card), as X-Plane can, in many areas, rely on the GPU more than the CPU. The Nvidia GTX 10 series and the newer AMD graphics cards offer more affordable HIGH quality performance.