One thing you might consider especialy is you are wanting to do this on the cheap without a lot of right anbgle gear boxes and drive shafts to get the power from under the engine to the grinder is to.
First off make an arm to extend out the front of the mower on a pivot which will allow the grinder to be raised and lowered. V belt drives can successfully be made to change their rotational plane from vertical shaft/ horizontal pulley to a horizontal shaft vertical pulley providing the extension arm is of sufficient length.
the grinder head will require a couple of bearings a shaft, and a housing to attach the pulley on 1 side and the grinding head on the other. a couple of idler pulleys may be required to insure that the belt does not come off at either end. a means of raising the attachment like that of the mower deck another means of de-activating the grinder (IE clutch) can be accomplished by shortening and lengthening the control arm.
I will draw up a generic model to offer a better explanation of what I am trying to convey.