Its perfectly possible to build, yourself, a machine for milling steel.
99.9% of people who start, fail.

I scratch-built a VMC.
It went like this.. (as my wife reminds me..)
A couple of weekends and some hundreds of €€...

((Router->Bridgeport M head-> ISO30 spindle)).

15.000 work hours later, I am finishing the V3 iteration of what is now a commercial quality VMC.

This is what it entails;
32 mm ballscrews, 1200 mm working length
Moving table, in steel, 1600x500 mm, 60 mm thick, 200 kg in mass (will double to 400 kg and 11 cm thick)
35 mm linear rails and trucks
ISO30 spindle (about 1300€, they are not that expensive)
Brushless AC servos, 400W, 1.3 Nm continuous, 3.9 Nm peak
125 kHz hardware controller for high pulse rates

Current step size is 0.2-0.3 microns on each axis.
Machine is 2.3 wide, 1.6 m tall, 2 m deep, approx 2000 kg in mass, all built in basic local tool steel "F1 calibrado".
Table size is 1600x500, with 1200 x 1200 work area.

Most people wont have the patience, time, space and ability to continue for so long, until you finish something like this.
I am now a professional machine builder, with factory training from a major manufacturer (2 years ago).
My own machine costs about 20k in components.

Retrofitting an old manual metalworking mill is much easier and cheaper.
A scratch built machine is much better (more work envelope for cost and volume, more rigid for the mass and size).

Current machine rigidity is 17N/um, which is acceptable industrially.
Goals are 10-70N/um rigidity according to industrial guides and textbooks on machine design.

A good set of retrofit gear costs about 2500€ and up.