Hi Atv
Personally I would go with a belt drive. I think it is easier to make properly and more technically correct. The ratio has to be reduction and has to be calculated based on your speed and load requirements.
To determine the motors power have a look here
http://geckodrive.com/support.aspx?n=493148But I think they are missing the friction element in their formula (though may be it is taken into account in their normalizing constant), so I'd add to the heaviest weight 15% before putting it into the formula.
The ballscrew pitch, again, has to be calculated based on your speed requirements and has to be suited to the belt drive ratio and motor performance.
I'd say start with the motors you can find (not sure you have to be using servos for a small machine, unless you are willing to spend more money and time on servos), and then choose the right belt drive and ballscrews to achieve the performance you want.
Daniel