I believe traction control is enabled in every ECU, and all you require is the switch and pigtail harness. I am not sure of this though. One of the vendors sells the OEM switch. I think it is BOE/koldfire or perhaps radium.
If you are planning to shoot for a national trophy I am sure you will need lsd/tc. Otherwise, a big front sway bar does alot to keep the inside wheel down.
My elise did not have an lsd and the front sway bar changed it from a one wheel peeler on tight turns to the point where I never had traction issues.
Seriously, my car is small
07 Exige-S, "Landshark"
05 Saffron Yellow "bumbleBee", on to a new owner
84 930 turbo, 06 WRX Wagon