You can get some deals on used 3-in-1 machines because people are generally disappointed with them. Grizzly used to have a small one that had a mill column that was removeable. Someone sold the base separately, so you could end up with two machines. It was the same as the mid-size Harbor Freight mini mill (all were produced by Sieg Industries:
http://www.siegind.com/ ). Unfortunately, I don't see it listed at present. It was a pretty good deal.
If you want something used, check out the For Sale section at
http://www.practicalmachinist.com/ You might want to list what you want and maybe someone has something. I learned of the Abene mills there. Someone I know from another forum bought one because he thought I would want it, so now I have to go to Rochester, NY to get my $100. mill All 2,800 pounds of it
There's a lot of info here on various equipment types:
http://www.cnczone.com/forums/
Some people prefer to get machines from Grizzly.com instead of Harbor Freight because of better customer service and parts availability. Grizzly's machines may be a bit better in quality even if they come from the same source in China.