its a good idea to do lots of drawings and mull over them at leasure

I predict from your drawings that you will have lots of fexibilty troubles resuling in a really disapointing experience (unless your cutting polystyrene)

you NEED as much ridgidity as you can pratically get without resorting to cast iron or stone lol
tacticaly you should be thinking about minimising flex EG your Y axis disecting the width of your machine and not the length and your Z axis travel seems abitious.
... imagine your Z axis at the bottom of its travel, if you put some pressure on it in the XY plane i imagine it would flex millimeters rather than tenths of millimeters:sad:

hope im not putting you off to much but get the job right and you will be doing cartwheels down the street when its done.... get it wrong and it will break your heart having to re-work the bugger... good luck