Sand is as you know extremely abrasive. I filled the base of my Chester Lux mill with the the B&Q variety sand and some gravel mixed in with casting epoxy a while ago too. (I am sure you can find cheaper alternatives to that though like roofing resin?). Then went to drill it to put linear rails and found that my drills would burn as soon as they broke though.

My advice is to drill and tap before filling. Then plug the holes using a hotmelt glue gun.

The epoxy/sand mix does not flow well unless it is more on the epoxy side of weight ratio. I think I did 10% by weight that produced good results but it had the consistency of mortar.