http://iphone.ebay.com/Pages/ViewIte...20diff&emvcc=0
Thats the one you want yeah- I think its cheaper because it needs assembling vs being ready-made. I'd assume the bearings are the same/correct size, though you can always look that up on the exploded views or part listings for the vehicle they come from ( lightning or D8 ).
Quality should be fine, cant imagine they would wear out very quickly even if they werent hardend steal, unless your kids drive like loonies...
7k front 3k rear yeah- Im running 7k & 5k but thats because diff tuning is not my specialty- just looked up a fairly generic setup and went with that- either one is a good starting point really.