I've heard quite a few good things about the old Tree CNC mills. I just purchased one, myself (Journeyman 330). It had some bugs in the control. Bought it on fleaBay for just over $2500. Fortunately, I was able to fix the control by replacing a couple electrolytic capacitors. I've yet to run the machine, but the fact that it works makes it a good deal.
Anyway, enough about that. What is the story on the machine that you're looking at? Is it up and running? Have you seen it or talked with the current owner? The original controls on the machines are getting up there in age (mine is over 20 years old). Upgrading/updating the control would probably be quite costly and labor-intensive. If the machine you're looking at is in good running order, it is probably worth considering. The Trees have a good reputation. Tell us more.