I looked at converting a small vertical milling machine. Mine was a Sealey model but as far as I can see it was very similar to yours in spec.
https://www.amazon.co.uk/Sealey-SM25.../dp/B000RO681O
I had already fitted ballscrews and steppers to another project and was looking to turn this machine into a dedicated CNC mill. When I stripped the table to look at the leadscrews I came to the conclusion that all was a bit compact to shoehorn ball screws into there. The existing screws were only supported in a single end bearing and the crossheads so unsupported at the outer end . Not sure if that would suffice
I wonder what sort of gear is fitted into the CNC variants of these machines. I think it would be good toget a peek in order to get an Idea what you may wish to replicate.