Mill/Drill recommendations

Hoping the experience here can help me make the right choice.

I currently have one of the small grizzly mini mills. It's OK, but
of course I had to re-do the screws, and it's pretty small. I'm looking
for something bigger, that has a decent CNC upgrade path. I've got
the electronics and motors to drive a decent sized mill, but I'm trying
to
get some suggestions here.

I do parts that usually fit on the grizzly with some inventive tooling,
but
would fit on a bigger machine without tricks. I'd like to get a machine
that has decent enough screws in it that I don't have to replace them
just
to use it. As much as I would like to bite the bullet, I can't get a
bridgeport
or other full sized mill right now, as I don't have the space, floor, or
power
to support one.

I've been looking at some of the ENCO, harbor freight, shoptask, etc
machines.
I really need table movement of a minimum of 12" by 7" or so.

So, can anyone share their experience in CNC conversions with some of
type machines? Good places to look for used ones? I'll buy new, but
I don't mind a deal. Anybody have experiences/methods (good or bad)
converting some of these Z axis methods to CNC? One thing I'm looking
for is reasonable smooth/solid ways/gibs and screws. I don't need
ball screws, and I'm no fiend for accuracy, but it should work
smoothly without binding or play.

Along those lines, anybody know of any good quality tables I might
retrofit to my grizzly. The grizzly really has all the power I usually
need for most parts, but not the travel (and I'm not all that impressed
with the linear motion).

Thanks folks.
John Murphy