Car pumps are much less than 12 HP - look at the size of belts driving them. (they are in the 1-2 gpm range). But they do have the advantage that they can be belt driven because their bearings can take side loads. Your spindle motor (mentioned right at the top) can't take side loads.
You can save the price of a VFD (and the electrical noise) by buying something like this:
http://hydraulics.eaton.com/products...VN-TM002-E.pdf
Variable displacement, vane, quiet.