12th scale is alot cheaper than most forms of racing out there, maybe save dirt (that is if you dont spend money on new tires and such...).
As far as cars... the corrally is a nice beginner car however, the front arms are not very durable (i have seen guys break alot of them at my home track) and finding parts and an arbor for a truer can be really hard. If you local hobby shop stocks them, then its not a bad choice
In the more competitive side of things... There are t-bar cars and links cars. I would suggest staying away from t-bar cars since t-bars are easily broken and can become tweak or damaged without you knowing and it can make the car completlty undriveable. Links cars are consistant and nothing really breaks on them. Such examples are the Speedmerchant Rev. 3 and CRC carpet knife. They cars are somewhat similar. The Rev. 3 comes with the old style front end which I highly recommend over the dynamic strut espically for beginners. The strut is a royal pain since the molds are very inconsistant and it requires alot of work and expeariance to keep it in correct working order.
Servo's are all around $60 you defently want to get the correct servo for your car, and probably a new one. Some can become damaged and really make your car screwy.
As far as diffs, I actually disagree with Impact. Fiberglass axles can creat alot more problems than graphite ones however, i am pretty sure that all current kits come with graphite axles.