The Losi kits actually come with a very nice setup out of the box. If you plan on racing, the first thing I would upgrade would be the tires. Find out what works good at your local track and get a set of those. The Losi rtr's come with very good electronics as well, but there is nothing that can compete with a high end servo. Your car will respond quicker and turn harder, so that would be the next thing I would recommend. My personal favorite servo is the KO Propo 2344 hi-torque servo. Otherwise the Airtronics 94358 works well to. A servo upgrade will be somewhere in the $100-$115 range though unfortunately.
To start, I would recommend just getting some tires and extra parts such as glow plugs. Overall, the Losi RTR's are the best bang for your buck that you will find, so I would wait on too many upgrades until you know what exactly you want to change on your truck.
I hope this helps. Feel free to let me know if you have any further questions.