UHID --
Totally confused on this one. I see it comes with several harness choices and I am guessing I would need the 32 wire one? Then those wires attach to the pots on the wheel, shifter, and pedals as well as the buttons on the dashboard. Then those buttons would need to be mapped in the UHID config utility and then in each emulator's settings to get the games working?
I would assume the wires from the harness just solder onto the pots directly
I didn't even realize he was selling harnesses.

Yeah, pretty much everything gets wired straight to the U-HID and it will report it as whatever you want, but it needs to be configured. (gamepad button, gamepad exis, keyboard button, mouse, whatever.....) You also assume correct that the wires just go directly to the pots. Nothing extra is needed.
G27 - yes it would be nice to play things like Pole Position, Turbo etc but I think we could make do w/o them in order to get games like Daytona, Cruisin', Outrun, etc
While PC games would be nice or even PS3/XBOX games this needs to be simple and easy for others to use hence the arcade games. Arcade drivers you can walk up sit down and play (just pick your auto/manual, your course etc and off you go) whereas PC/PS3/360 games will require you to level up, configure the car etc etc etc and guests don't want to bother with that like I would.
Thus it won't be as user friendly and thus not get used for parties (which we have a fair bit of)
The G27 is probably overkill if you're just going to use it for arcade games.
There aren't any arcade games working in emulators that used a 6 speed manual except for ridge racer and it supports paddle shifting.
I also can't think of any 100% working games that require a clutch either, except for SF Rush, but it takes an extremely high end computer to run.
Ridge Racer had a clutch (only a switch, not analogue), but the emulator works properly without it.
My advice would be to buy a much cheaper wheel that only has 2 pedals and mount it behind the original dash like Gbeef did with his G27.
Hacking the arcade pedals to it is easy. Just swap out the pots with ones that match the value of the ones in the PC pedals,
then move the wires from the pots in the PC pedals over to the ones in the arcade machine.
Then hook the arcade buttons and shifter up to a keyboard encoder and you're done.
That way people aren't confused by the 6 speed or extra buttons.
The easiest controls to understand are the original arcade ones, preferably with labels.

Check out how I did my buttons. They're pretty self explanatory, except for people not knowing V.R. is for View Change (sega's wording, not mine)
If you want plug 'n play though, the G27 will have you covered.