I would very highly recommend not to do a CNC retrofit at once.
Since You would probably do a terrible job in multiple ways for multiple reasons both economical and technical.
It is not hard/difficult to do a *good* cnc retrofit.
But quite a lot of work.
I have done rebuilds upto modern industrial levels.
But there are endless second-level gotchas, not documented anywhere.
And 99% of problems are only really solved with proper hw that is NOT what you expect and is somewhat expensive and has its own set of second-level dependency issues.
So whenever you save 100$, or 200$, per axis, you probably cripple 98% of the good stuff later on.
Recap:
Don´t use steppers.
Use ac brushless servos, 5000 count encoders or up.
These need a good hw controller. How is the support, docs, samples, faqs ?
Use an ac servo spindle, not VFD.
You need 50+ IO lines on the controller, 7 per servo, +/-. + 25 IO lines for (each) MPG. Etc..
Poor:
One can make a dirt-cheap stepper-cnc mill ..
with terrible resolution, holding, rigidity, acceleration, no mpg. (poor Speed. Mostly irrelevant).
Better:
Making the thing 5-10x better costs only 100-150$ more per axis, plus a bit.
And 30$ in steel and some hours of work, per axis, per component, 2-4 of.
Plus some 30$ bearings.
Skill.
Research.
Fab work.
To get good results you need industrial stuff like linear guides, or automatic oiling on gibs/ways (cheap to do).
Screws need more rigid mounts. Tensioning ideally. Ballscrews ideally. Thicker screws.
Double nuts. Oiling.
Big couplers to servos, or rigid couplers and very very good adjustment and setup.
The components as such are not very expensive, per piece, but probably cost a bit more than You thought.
And you need more of them, and they need to be bigger, and you need to build stuff like more rigid mounts and screw tensioners, very easy to do, cheap in materials, but the hours add up.
Chip shields.
E.
400W AC servos, 220V, 5000 count, 3000 rpm, cost 300€ with 22% VAT here in Spain, EU, full kit.
Steppers for decent results cost about 200€. (80V+ chinese steppers+drives).
The servos are more than 10x better.
Anyone using steppers for work/work is mistaken.
It costs about 4000$ in stuff for a good cnc mill retrofit.
500 kHz+ controller with high speed hw/fw support for probing, limits, tapping, mpg, servos, spindle.
4 axis, 100+ IO, 1 MPG, boxes, io lines, cables, conduits, ferrules, crimping, 24V DC (ideally) IO PSUs.
Cable conduits, guides, passthroughs.
Signals for each servo for hold, fault, enable, inpos, z index, and usually +/- differential signalling.
Couplers, mounts, sheetmetal, paint.
4k is for 3 axis small servos, 400W.
Add spindle, 2.5 kW == 2000 $.
A cheapie stepper driven Bp can be made to do good/excellent work.
Very slowly.
Belt drives for high resolution, special posts or hand coding, probing and macros.
Tool selection and multiple finish passes.
Pretty good accuracies, small feature sizes, and great results are possible.
With skill, experience, and often adjusting stuff multiple times for good results.
It´s somewhat finicky, and very slow and unproductive.
It is just barely possible, but hard and slow and needs constant fiddling for excellent results.
My experience and opinion is that making hand sized parts of high accuracy and good finish, in steels, is about 5-10x more productive with the "good" retrofit than the "cheap" retrofit.
Rigidity is everything.
