The K153 kit isn't that bad a deal. It provides all the ancillary kit you need (charger, charging sack, radio, etc.) and gives you a great start in GT12. You will need both cars to have differentials fitted as the solid axle will make driving the car frustrating.
It's worth going to your local model shop, or finding one on line you can call (AMC - Action Model Centre - for example) to try and do a deal on a Superstox GT12 S1 car (it comes with the diff), radio, servo, body, charger, charging sack, etc. You can then get the electrics mail order from Hobbyking (see STM's post above) as those prices you won't find in a shop. Be aware that the servo needs to be a Core one as the usual Futaba, etc. stuff doesn't fit.
More of your money will go on tools, tyres and bodies, a few pinions and other pit-related stuff to get set up. Perhaps you could price up a complete car in a few different ways and then come back here with your list. We can let you know about the stuff you have selected and provide more help if needed.
Best of luck - as STM says this is one of the best classes of RC racing ever invented! HTH
