What iron to use in a retrofit???

I am looking to add a cnc mill to my small machine shop. I would love
to go with a VMC but limited on money I can spend. What machine would
be best for a retrofit? I am planning on adding possibly a Centroid or
Ajax control to it. I am looking for a full size machine that is rigid
and accurate ,planning on using this mill to add cnc capability to my
shop. When I say full size and rigid I mean not a benchtop mill , I
think as far as rigid , a knee mill would be ok it wouldnt have to be a
bed type mill. Out of all the bridgeport Boss machines does one
outshine the others as far as a retrofit is concerned? How about an old
Hurco or Tree? I am at the research phase right now so I appreciate
all input.