You can keep it simple by just using the pump without a grant or filter as the grain bed will work as the filter. The method you use for an inlet screen in the bottom of the mash tun will determine how fine you can crush the grain without sticking the mash. A worth while option would be a flow indicator in the pump outlet that will let you monitor the flow and set flow rate with a ball valve on the pump discharge. Here is a flowmeter from Dwyer that should work, VFB-81-SS with a flow range of 6-60GPH (.1-1GPM), unit cost is $47 and shipping from Dwyer. The fine grain particles that flow through when you start do not seem to bother the march Mag-drive pumps, and are usually cleared in a couple minutes of recirculation.