[jimmy]

The WE has the bits you really need which are the pulleys and belts - the alloy stuff isn't important but it's always nice to have. A one way diff helps the car turn and pull out of corners - it takes a different driving style and suits track where you're not really having to use the brakes much / at all - you have to drive in a more flowing style. I prefer to drive with one - the only time I took mine out in the 501 was at the VAASA track in finland because every now and then I just couldn't recover from getting the nose high on the huge jumps so had to swap to a diff.
I preferred the smaller pulley up front - on the WE I think that it's a 37t diff - so the 36T one way helps calm down the car whilst still turning nicely.

Centre one way gives you no front brakes again but allows the car to rotate more because the rear wheels are getting slowed by the motor whilst the entire front end is coasting. With the diff up front and a centre one way, it's just like a full-time 4WD on power.