servo motor for CNC conversion

I converted an old "ELOX" EDM machine to a CNC milling machine using
stepping motors. Workks great except that the max. feed rate is
slower than we would like.

I now want to convet another type of machine, but this time we want
to use servo motors.

I read somewhere that you can use most DC brush motors, by adding an
encoder, as a servo motor. Is this accurate? If so, any
recommendations on a source of commercially avialable motors? I hate
using surplus in case they have to be replaced some time in the
future.

For the drivers is the Geckodrive G320 the best bet or are there any
reasonably priced altenatives?

If I wanted to change to ball screws do you have any recommendations?

Thanking you in advance for any info,

Best regards,

Stu