I think you'll be lucky to get 1000 x 600 out of the current size, wait until you've drawn up the gantry it's surprising how it grows My router is 1300 x 750 and I'm not going to get that much working area, close but no cigar....Ballscrews, for wood you need speed so don't use 1605 ballscrews use 1610, if you need precision then you can always reduce down with pulleys (I'm assuming that you're connecting the stepper to the pulleys with belts to reduce the resonance?)

Gantry, L shape is fine either 2 lots of 100 x 50 box section or C section 20mm aluminium plate, the box section is the easiest and cheapest but you'll need to make sure that it's flat and twist free. If using tooling plate and if machined properly then this is virtually eliminated. Why not use 100 x 50 x 1/4" (10 gauge?) aluminium in an L shape, EddyCurrent and Kingcreaky have both used this arrangement and it's a very neat solution.

Get the gantry drawn up, you'll find that your design will change, and then change, and then change.........