Hands down, the adjustable turnbuckles. I'd also recommend better connectors - the Deans Micro 2R plugs are good. At least 3 pairs are needed for a full conversion.
The stock steering servo is good enough for general bashing. Just make sure the gear lash is set properly, and that all the parts are there (a quick visual once-over is enough... you can do this while the battery is charging). If it is missing parts, Associated is good about sending you whats' missing.
Outside of that, I'd say just beat it like a mule until something breaks or wears out, and then upgrade from there. Get your moneys' worth.
I like mine; all I've replaced on it so far is the turnbuckles and connectors. Brushless motor and lipos' are on the horizon.