Someone once told me the answer to "how big a (lathe, mill whatever) should I buy?" is, "you'll always need one about 1" bigger than what you have".
For what it's worth, my rule on tools is to get one that seems like a good combination of features for what you can think of now. You'll always want to work on something bigger.
Yes, I have a G0704. Converted it to CNC and have used it on projects well within its work envelope, so far. I thought it was a pretty decent mill right out of the box. I already have about the same cost in the CNC additions as the original machine, and I'm looking at a new spindle. It's not that the spindle is bad, it's just slow for a CNC mill and I'd like to be able to control it from the CNC program.
FWIW,
Bob