Is it the new version that comes with a LiPo battery? If not, that is a great upgrade (you will have to get a charger too however).
The single most worthwhile handling upgrade is probably getting rid of the servo saver (RPM and other companies make a solid replacement bellcrank). This will probably strip the stock servo soon afterwards, a cheap and good upgrade is a Power HD 1501MG. Nothing comes close at that price as far as I know, though obviously if you spend more there are savox/hitec/futaba options which are faster and/or stronger.
Durability wise, many people have problems with the shoulder screws coming out of the front spindles upon crashing. Aluminium spindles (hot racing or ECX themselves) solve this issue.
For more speed and runtime, you can put it a brushless esc and motor. Ebay combos of $30 work reasonably well, more expensive sets from reputable brands are better (and come with some warranty in most cases).