My 1987 Taiwan Webb 4VH 10 X 50 mill (3,500#) came to me with ball screws and servo motors but no computer. It had been a factory CNC modification that had quit working, The machine was very sound and I refurbished it with manual controls but left the ball screws in place. The X axis has about 7 degree marks of backlash total (3.5 each way) and the Y axis has 0 backlash. The machine had been used so that's where the X backlash developed, I'm sure. On the good side, even after 7 years of constant use and lots of projects, the backlash hasn't changed a bit.
I'm not the resident expert on ball screws but i suspect that even new ones might have some small amount of backlash. That's just a guess, though. .
In any case, I've never been sorry that the machine has ball screws. The Webb has dual table locks on the X Axis and I always keep some drag on them or lock them down totally when milling. The milling process has yet to drive the table even when climb milling. I'm quite pleased with them all in all.
The bigger issue in converting an older machine to ball screws would be the overall cost. I think it would be easy to exceed the value of the machine when doing such a conversion since everything has to be changed.
I listened to all of the negative talk about ball screws on a manual machine when I first got it and considered converting it to standard acme screws. With all of the parts from Webb, it would have come close to $1,500. which is about what I paid for the machine when I got it! LOL
Good luck on whatever you decide.