I installed the 4-axis DRO (TPACtools.com) on my Jet JVM-830 mill in a couple of days. Did the X-axis with shop made bracket on the front of the table T-slots. Had to make a new central bracket for my power feed limit switches and adjustable stops. The X-scale is spaced away from the table about 1" so the power feed switch, stops and table locks are still accessible. So, bolt on, no drilled holes into the mill.
Y-axis for me went much the same. The JVM-830 has a T-slot track with adjustable stops in the Y also. I made up brackets that mount to the track to mount the scale.
Z-axis was a bit more of a challenge. The mill column is tapered, so made wedge pieces that bolt to the column. Then adjustable brackets that attach to that. I mounted the Z-axis scale with the opening to the back to keep chips out.
4th axis was a digital vernier caliper on the spindle. Made a couple of bracket for mounting it, probably took 2 hours. All of my bracketry was made from 1/2" thick aluminum.
The work is well within the capability of hobby machinists. If you're not there yet, it's a great learning project. I consider myself to be at an apprentice level; all self-taught. I still make mistakes all of the time, but have completely gotten over my fear of making mistakes. That's how you learn. If you post your mistakes here you don't have to worry able being made to feel stupid. No egos here.
Bruce