The fact that USB flight input devices appear to Windows only is not a limitation; for Mac most are programmed through the sim itself. I know X-Plane does that.
I've simmed a lot and know that good sims need a lot of processor and video hardware horsepower, and you'll also need a lot of RAM. They also run up the internal temps. A Mac Pro has better cooling than an iMac. You can get a program like SMCFanControl to manually set up the fans speeds before starting the sim.
I know of no sims today that will still run on a PowerPC Mac (2005 and earlier) so you need to look to an Intel Mac with top specs. If looking at iMacs, start by looking for the quad-core i7 processor. Those models usually came with an up-rated video card. Thing 12 to 16GB RAM.
My 2010 iMac has the 2.93gHz i7 processor, video hardware (ATI 5750) with 1TB VRAM, and 12 GB of RAM. It runs the x-Plane 10 demo fine but I've not bought the full version because v10 seems to make foot pedals darned near mandatory, and I don't want $100 worth of pedals under my regular work desk at the moment.