To add another point to the good ideas so far...
Todays cars are so technical, that it helps to stick with the same one for a while. But, in this competitive market, new cars and up-dated cars come out constantly. This makes gelling with one car difficult and (in some cases) parts obsolete ($).
From this standpoint, the Schumacher MI3 would be a good choice. Being the most adjustable car on the market means it will be competitive for a long, long time. Plus, it's the easiest car to work on (ie diffs out with two screws, spur/layshaft w/ one...).
Team Schumacher - Team Powers - EDS Specialized RC Tools - RCSpeedshop.net