Our team went through this with $4000. we ended up buying a bridgeport.. but here are the other options we considered
1. Sieg X2
http://www.grizzly.com/products/G8689
2. Sieg X3
http://www.grizzly.com/products/G0463
3. ZAY
http://www.lathemaster.com/HEAVYDUTY...E%207045FG.htm
4. IH Mill.
http://www.industrialhobbies.com/
5. Used bridgeport
www.craigslist.com
We went with 5, why? we found a great deal for a very un-worn bridgeport (nearly pristine) with a good DRO, power feed, full set of precision collets. With a vari-speed head.
IMO the X2 has serious rigidity issues, but its good for whatever work FIRST requires (+-2~3 mil) and you can take it to regionals (reasonably)
X3 is much better, more rigid, and people have supposedly held mil tolerance.. so I guess its pretty good there
the ZAY and IH are by the same manufacturer, the IH is just HUGE and has hardened gears, as well as a CNC option. the free shipping also makes it worth it... we were about to go with the IH mill when we found the bridgeport deal.
Honestly, just pick a dovetail column mill, its more rigid, doesnt need re-center every time z is changed... or a knee mill (better, but more expensive).
Also, I'm not sure if used equipment is the best way to go.. fixing the machine can be as much of a challenge in and of itself..
Tatsu