Are you using the PC you've got in your sig or setting up a new one?
Basically have a look at the recommended specs for Iracing and go off that plus a bit more. But usually any decent gaming rig built within the last couple of years will suffice.
If you're going 3 screen its cool to get them mounted with a play seat or something similar. Depends how extreme you want to go. Check out Pagnian Imports for a few cool options.
If you want to go more budget you could just slap the screens on your desk and buy a wheel stand pro, which holds your wheel at a good height.
As far as wheels go you've got a few options
Logitech Driving Force Pro - cheap but still good and get the job done
Logitech G27 - more expensive but better quality components, force feedback and a H pattern shifter with clutch.
Fanatec RS blah blah with clubsport pedals - the cream of the crop IMO for racing wheels.
I currently own a G27 and it's great. If I had expendable money though I'd definitely upgrade to a Fanatec as it's the only 1 of the 3 which also supports xbox 360. (though sounds like you've already got a 360 wheel)
If you want to be sociable while driving, its good to get a headset. It's quite tough to type while driving. If you do try to type, you'll usually end up typing sorry mate, as you slam into another car and take out half the field. There's some serious dudes on Iracing and they wont like you for it
If you sign up to Iracing there is a huge amount on info re: setups and specs on their forums. Just go searching about.