For what it's worth, neither of the practice kits are particularly applicable to building an RV12. They predate the RV12 by years and are designed to teach rivet gun riveting along with some very basic sheet metal skills. The tools called out for the practice kits contain pneumatic rivet guns, bucking bars, back rivet plates, etc. None of these are needed or useful for an RV12 build.
Skip the practice kits. Don't spend any money yet. Take the advice of others here in the thread and find someone near that is working on a -12 and willing to share some time with you. There must be dozens of -12 builders in central Cal. near you. Van's can supply you with contact information.
A related question is what tools are necessary to maintain an RV-12 that perhaps you have bought and not built. In that case, a good reference point would be to look at an RV12 tool kit
and go down the list deleting what is pretty obviously not needed. Examples of not needed would be any pneumatic tools. And you obviously would not need over 300 clecos but you would need maybe a dozen in each size. Using the list as guidance you could probably order items individually as you identify the need.