Some may disagree but I would use the money and spend it on tyres and just practice, practice, practice. Weight can make a little difference but unless you can get around the track at a decent speed without crashing then all the weight in the world won't make much difference.
Try get the car set up without weight as a car with without weight should generally be quicker, adding large weights at the back like the 2 you have suggested can cause the car to sway more and cause the back end to come round on itself. Cream on here does some some weights which replace the lipo stops 20g for front and 20g for rear which doesn't put the weight as far back, could also try them but depending on your skill level at the minute would definitely spend the money on tyres.
