I could use my current machine to play. It would probably require, at the least, a RAM upgrade. Playing from my 22" monitor would be a serious step back, after using the 42" TV. Then there's the logistical issue of either moving my G27 rig to the computer, or, moving the computer in place to hook to the TV, which would require, with nothing but the RAM upgrade, hooking up the DVI and the sound card output to the TV or receiver, then, moving it back, when I wanted to do computing stuff.
The other option, would be to build a, essentially, bare bones box with the necessary hardware, just to play iRacing. Me being me, I wouldn't go with the minimum requirements, but rather, I'd exceed the recommended, at least in processor, RAM and Vid Card. While I've not specced out a machine in a few years, I'm pretty sure all the parts would run up well over $800.00.
I've bought and built computers with one game in mind, but in all those cases, that computer pulled duty for much more than that one game. I don't even need my wife to object to the latter plan. I can't justify it.
iRacing is for a much more serious and/or dedicated group of sim racers than I.
On a side note, last night I watched a program called Horsepower on NBCSN after F1 FP1 and have to say, I was fairly drooling over the sim rig FA and KR get to use in Maranello. Wonder how much RAM and processor that thing uses?