Toroid Inductor's Copper Thickness and Frequency Question
1) The copper thickness does not affect the inductance. It will have an effect on the DC resistance, due to its cross section, and on the RF resistance, due to its surface area.

2) There are at least two limitations on maximum inductor current, the heating effect in the copper, and the maximum field that the core will handle. Above a certain field, the permeability of the core plummets as it goes into saturation. This means the inductance drops by one or even several orders of magnitude.

3) If you use a toroid rated for 50MHz to do low frequency filtering, it will work, but it will not have as high a permeability as one rated (for example) only 1MHz.