Thats a good idea by over hanging the X rails. I guess you could also have a removable section in the centre of the bed to do the same thing, but will obviously limit the size of the material that is being cut. May take the idea up and incorporate it into my design. What length overhang would you say? diagonal supports could be added for a bit more strength too.
Im debating over the Z axis travel now too. i originally stated 300mm, what is the norm? or will it be best to go with less around 200mm. I will be mainly profiling, but will occasionally mill in 3D (modelling foam), if pocketing im guessing the depth of Z travel will be restricted by the tool length?