You should be able to fit the gantry into 280mm, maybe 300mm, quite easily, especially if you let the Z-axis stick out at the front. On my machine the X-axis bearings are 300mm from end to end, which is adequate. It's certainly good to increase that if you can, but not absolutely necessary. The same is true for getting the centre of mass to lie precisely between the bearings on X.
Click image for larger version.  Name: My Gantry side view.jpg  Views: 1107  Size: 73.1 KB  ID: 7310
You can make a sufficiently strong Z-axis 160mm, maybe 150mm, wide so the frame doesn't need to be wider than the Y-travel plus 160mm. Again, if you want to make the Z-axis wider and space the bearings out more that's a good idea to get better rigidity.

Nema 23(/24) motors will be fine for the Y and Z axes. For X, try using irving's spreadsheet:
http://www.mycncuk.com/forums/faqs-p...do-i-need.html