I have an imperial M250 so fitted a DRO to make the machine more user friendly (the graduations on the handwheels are just plain confusing). The scales I fitted were from Machine-DRO.

The longitudinal scale is optical so quite bulky but there is plenty of space at the back of the bed so is the cheaper option. For the cross slide I used magnetic scale fastened to the tailstock side of the cross slide because being only about 10mm wide has minimal effect on how close the tailstock can be to the carriage. (magnetic strip is let in to the ali extrusion visible in the picture.

I cannot recall whether I had to drill and tap any holes in the cross slide (but it would not have bothered me anyway) but I had to remove the ball oiler fitting so that the read-head mounting block could be mounted easily. I remove the head when I need lubricate the carriage.

The M250 is good machine and I'm sure it will give you good service, the main annoyance from my point of view is the need to mess about with change wheels nearly every time I do any screwcutting (and this is a machine with a screwcutting gearbox!

Ian P