Hi
Not sure but does your kit comes with 2.4ghz tx/rx? , battery wise you need to get hump pack like this
http://www.ebay.co.uk/itm/6V-3300-5000mAh-SubC-SC-Premium-Racing-RC-battery-pack-with-custom-connector-/231500864010?pt=LH_DefaultDomain_3&var=&hash=item35e686da0a
Size and capacity depends on your buggy design and where you can mount it, if you using 4 x aa battery that could be your problem why the car shoots off, also if you can set up your fills safe so the throttle servo goes to natural or brake position once you lose signal, failing that get fastrax return spring
http://www.modelsport.co.uk/fastrax-throttle-return-springs-for-.21-28-size-engines-2-/rc-car-products/371962