OK I'll start the ball rolling with a quick reply.....I'll certainly not get into the debate about using MDF.........

The Y axis on the gantry needs moving to the back so the spindle is nearer the middle of the bearings, not hanging right out at the front. You'll also gain some more cutting space this way.

You need cross bracing on the legs to stop the machine twisting.

I'd beef up the gantry to X axis bearing mounts as a lot of force goes through them...

The X axis (long one) rail supports look a little flimsy, Ideally I'd put them on the torsion box as they'll be more supported that way and won't flex litle the sides you've currently drawn. That sort of design is great with a steel frame but not in wood.

The rails on the Z axis should be on the front plate as this will add to the strength, the bearing blocks should be on the back plate.

The SBR rails on the X axis should be 20mm ideally for strength.

Ballscrews should be 1610 for X & Y and 1605 for Z.

Motors and drivers, without knowing what sort of budget you've got.....

Motors: http://www.cnc4you.co.uk/Stepper-Mot...x-3-60BYGH301B

Drivers: http://www.zappautomation.co.uk/elec...ion/em806.html

These will be able to be used on your Mk2 machine, don't forget you'll also need a break out board, power supply (70V to get the best out of the drivers and steppers), Ball nut housings, E stop, limit switches, motor mounts, shielded 4 core cable (lots more than you think!) couplings (best to use pulleys and belts to lessen resonance) loads of nuts/bolts/screws/washers and lots of beer...sure there's more but that'll do for starters :D