I am assuming that you are not talking about racing at the track in a roar legal configuration if you are trying to race 1/8th. If that is the case you will want a brushless system that is sensorless and can handle more than 7.2 volts. Some guys have been running systems with 11 - 14 volt lipo setups that are insane. I know there is a big brushless section on the Radio Control Zone forums on brushless. But you are looking at $$$ to build one. You will need a lipo charger \ lipos\ ESC and motor. It better be one big track.
For a roar legal system I would look at the sensored brushless system from Novak. They have a 4.5 motor coming out. Personally I have a 5.5 and I can not put it down in my track for a 2wd. 4wd you can use it. You will also want some of the newer batteries. IB3800 will give you some great punch. Unless the track is huge I am not sure a 4.5 will be usefull. More motor will not make you faster. Even the Nitro guys can tell you that. The one that drives the car at the correct speed\line will win. Also I am not sure you can get a 2wd to keep up with a 4wd(assuming both are driven to their ability of the car.). The front wheels pulling is a huge advantage. You would probably need a 4wd buggy.
I would vote brushless over brushed due to maintenance of a mod motor. If you like to tune and tweak I would bet you can get a little more out of a mod for a 5 min run. But with the brushless you just clean the bearing 1 time a month and oil.