The 4'x4' machine is a good size and the kit you mention is ideal for it. If you want to use lead screws and not rack and pinion then use 2 start or 5 start ACME lead screws and anti-backlash nuts. 5 start turns slow enough that you can go a little more than four feet and still get good jog speeds without whipping. Anything over 4'x4' is the size where people usually decide to go to rack and pinion. There are many 4'x4' machines originally built with lead screws that have been upgraded to rack and pinion.
You will use multiple step-downs for wood, MDF, and aluminum cutting anyway, so that's something that you set up in the gcode according to the type and size cutter you are using. You can go deeper than 1" with the cutters that have flutes long enough.
I would recommend the very popular Hitachi M12VC router for this size machine.
CarveOne
CarveOne
http://www.carveonecncwoodcraft.com