Greetings Dan.
Il answer what I can, however Im no pro...
*You want 2 ballscrews driving the X on a gantry that wide, to stop it crabbing
*in regards to one stepper or two, this is a bit of a sore subject on this forum. Both methods have their advantages comes down to personal choice. Im going for one stepper driving two screws.
*(controlelr/breakout board) This is talked about in lots of different threads by people far cleverer than I. So I wont comment.
*extrusion...get a design drawn up, its the design stage that will answer that question. Its all very well me saying yeah 80x40 but it depends on your design. Mine uses two peaces of 100x50 box 5mm thick in a L setup
*The HiWin type cartridge rails are the ones to get.. They are expensive but from what I read are about as good as it gets.
The supported round rails from china are what I went with, which I understand are good enough (so long as you pick the supported ones) but nowhere near as good as the HiWins

I would read ALL the build threads... If you had done this prior to the above post you would of been able to answer all those questions yourself, from the wealth of information you gather from reading them. Then get some designs uploaded and be prepared for them to be picked apart!!!

:-D wish you all the best with your build. make sure you keep this thread updated as there are some sad individuals (me) who regularly check for updates to build threads!!!