The main modification I made is to install a 12,000rpm spindle at the front of the mill for engraving. This works really well. The standard spindle is way too slow for a small diameter cutting tool.
I swapped out the original main 5 amp fuse for a 5 amp circuit breaker. I found that I was blowing a lot of fuses and it was a bit of a pain swapping them. Was simple to swap out and fitted the same hole.
Changing toolholders is easy if you add a spindle lock. Just press in and use a ring spanner to loosen the drawbar. There is a guy who makes these for the M3.
A stand is of course a necessity! I have the mill mounted to some solid timber supports. That way it is a bit higher and easier on my back! Also, it is easy to get the the ballscrews for lubrication.
I added a large vice, and fitted an adjustable endstop. This is great for setting up multiple parts in the same location.
A tool stand holds all the required tooling, with the keyboard at the front. With CNC you accumulate a lot of toolholders because setup is much easier with a known length tool.
This is the usb pendant for manual control. A really handy item. I got one that has a USB connection. Wireless is also available, but I wanted to keep things simple.
A coolant system in a 20l plastic drum with an aquarium pump does the job.
I also have a stepper powered rotary table that I use for fluting rifle barrels, see video. To get a good finish, I cut in 1 direction only, at slow feed rate.