For me, the main parts to upgrade to WCE spec are the front and rear shock towers which are far less prone to delamination, the wide front bumper and as already mentioned, the slipper clutch. The D4 is a fantastic car, really nice to drive and responds very well to setup changes. I am sure you will be fine as a beginner at your club, everyone there was a newcomer at some point!
The parts you need are available from anywhere with a Mirage account but Apex and Modelsport are pretty good for bits. I would pick up some front wishbones, a couple of front shocktowers and some diff cases as spares.
The upgrade to brushless and Li-Po is definitely something for the future but certainly not necessary straight away, especially with an 8x2!!
Front one-way is something that you can think about when you go racing but for just running round a park then I wouldn't bother just yet.
Hope this helps mate.