I spoke to the Idaho Tax Commission today and they said as long as we fill out a sales tax exemption/transfer affidavit showing that it was gifted then I will not owe sales tax on the kit. Only the things I purchase myself from this point forward. So that was great news
On the statute of limitations here is what they had to say.
"Yes, Idaho has a 7 year statute of limitations (for persons not holding an Idaho sellers permit) and that would apply to your grandfather’s purchase in 1992. However, sales tax is a transaction tax. Thus each time a transaction occurs Idaho tax is due unless an exemption applies. Your acquisition of the kit from your grandfather in 2015 is a separate potentially taxable transaction except that it is exempt via the gift exemption."