I'm not sure what your experience is with these gages, but they measure the coating thickness from the bare metal up. They will not tell you how much clear is on a car, just the total thickness.
You can have a car with a lotta primer and very little clear, another car with little primer and lotsa clear and they can read the same.
They are a great tool if you know the limitations.
We use them in used car reconditioning, but have no way to really know when you are buffing off too much clear.
We also use them to check mil thickness when applying primers, base, sealer clear etc. In this case, we know exactly how much clear WE put on and can use the gage to tell us how much we can cut and buff.
We have the ferrous ETG-A from Pro Motorcar Products.
I believe ours was a few hundred higher than the one in your link, your's looks like a good deal.