Info: E revo advice for track tuning

I just entered my first multi class racing tournament with a 2wd rustler and stampede. They held pretty well but the only 4wd I could race with was my x maxx which is fast for how large it is but I need a truggy. I'm going to buy an e revo brushless and I would like to know from anyone with experience with one what upgrades to go with vs what to look out for. I was told I can't put a center diff in it if I race? Any help at all would be greatly appreciated. I'm trying to avoid any headaches before they may happen.

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