I mirror the RF45 recommendation.
These have R8 spindles like the Bridgeport.
http://charteroakautomation.com/our-.../cnc-bed-mill/
That said, I have a highly modified Sieg X2 and have used the stock Sieg X3.
These have R8 spindles like the Bridgeport.
If you get one in good shape they can hold 0.001".
http://www.siegind.com/products_list/&pmcId=29.html
http://www.hossmachine.info/
Tool changer
I also own a ShopMaster Patriot VFD post-2012 mill/drill/lathe that I got for $4,000 with shipping used complete with CNC.
My machine takes 220V and has 1 VFD and 2 drive motors.
These have R8 mill spindles like the Bridgeport, MT3 for the lathe tailstock.
These machines support rigid tapping.
They can have sensors put on both the lathe and mill to 'close the loop' for RPM through the control PC.
http://www.shopmasterusa.com/content...-1-machine-cnc
The latest model just sold on eBay for $5,600 because a school ordered some and the funding fell through.
Also I've seen Tormachs here and there in the $5,000 range.
http://www.tormach.com/
I own both a MaxNC10 and MaxNC15 CNC min-mills with 4th rotary axis.
These are like Taig and Sherline.
They are a good tool for small jobs and could be used for jewelry making and aluminum gear making.
I wouldn't necessarily make robot sides with it because the travels max out at 16".
http://www.maxnc.net/category_s/1817.htm
You want to make sure with the Chinese mills you can get support after the sale in case you have defects.
I hesitate to buy any used, 'been laying around almost new', Chinese mills because you don't know they were ever tested.
You don't need a mill deck the length of the side of your robot: if you put a guide on the back of the deck and dowel pins at the extreme travels. You end up with some extra holes but you can then support the over-length on rollers. Just tile the operations.