The M$ wheel already has Paddle shifters on it, anything past that is just mechanical and electrical work. The difficulty of it depends on how authentic you're wanting it or what kind of setup you're going for. Making a slap shifter is simple, as it's pretty much just a couple of buttons and technically works the same as the Paddle shifters do. Then a clutch is relatively easy enough, it's just a pedal that doesn't let the shifter buttons work unless it's pressed first, but going any farther than that, like making a 6 speed shifter for example, gets many times more difficult. That would require some coding and such, because the game isn't going to know what gear you want to be in or if you skip any, like go from 1st to 3rd, or 5th to 1st, so without some kind of PIC or AVR in there to do the math and then output the correct number of shifts to the controller it just wouldn't work. That kind of 6 speed shifter setup is really not worth getting into as the game really needs to be setup to use it, just too many issues that can crop up, and making it bulletproof in one game would most likely screw up how it worked in some other one, but it's all in the realm of doable.
The 'rev indicator' can't be a true indicator without some type of signal from the game being played as well, like how Force Feedback works. What you'd have to do there is wire it up to the Accelerator, so it's more of a 'pedal position meter' instead of an actual rev indicator. That can be done with a PIC or AVR and having the pedal's POT wired to an Analog input, then some coding to just turn on 'x' number of outputs to light up the LEDs depending on the voltage it reads. So the harder the pedal is pressed, the more LEDs light up. It's not a real rev indicator, but about as close as you can get.
There is also a blog going on about another F1 type 360 wheel conversion here. - http://xboxwheeldevelopment.blogspot.com/