I reckon the best bet is to choose a mill by sheer weight of iron and available space, then add ball screws stepper motors etc.

If there's no secondhand Bridgeport available nearby, the best 'of the shelf' deals were from Warren but I am probably out of date. I used their Major mill, which seems to have been derated somewhat since I bought mine.

http://www.warco.co.uk/productimages/documents/P61.pdf