Eddy is correct with dropping the SBR rails it will let the machine down badly. The Z axis is THE most important part of the machine so if weak it doesn't matter how good the rest of machine is the cut finish will be poor.
Also 300mm Z travel is quite a lot so if your not having a adjustable bed then you will struggle to cut anything hard and thin(<100mm) unless you raise the material to the cutter and reduce Z extension. If not you'll experience bad chatter and poor finish on hard materials and you can forget Aluminium at full 300mm extension.

Regards ballscrews then I wouldn't use 16mm diameter for the long axis because in my experience it's just a bit too long. Now you can get 10mm pitch in 20mm Diameter then your much better using them over 16mm at this length. The difference in performance is neglible and your far less likely to get whip so often can go faster than 16mm screws that whip.!
Use 16mm Dia 5mm pitch for Z axis.
Also I recommend you connect steppers to screw with timing belts and pulleys to help reduce resonance and add some flexabilty with ratios if required.

The gantry you have drawn will need some bracing on the bracket holding the ballnut or much thicker bracket.
To be honest I'm not a fan of the place you have put the ballscrew as it's in the firing line of chips and is fulnerable to damage being high up and exposed.

Electrics then I urge you to go with good digital drives, Esp if building from steel as they handle resonance so much better than anologue drives. Also if planning on using Mach3 then invest in a motion control card. Both these will give you the best machine possible and least hassle along with best performance.

3Nm motors run around 65-70Vdc will be fine but check out the indutance of the motors first, if buying from Zapp or CNC4You then you'll be fine but if from Ebay then they could be High inductance motors which will underperform.
The drives you pick will need to be capable of a higher voltage than 65-70Vdc you'll run motors at with a little safety margin so 80Vdc are the norm.

Toroidal PSU like eddy mentions is best for CNC and you'll need to size so outputs correct Dc voltage running drives at with Amps roughly 70% of total motor amp ratings IE: 4 x 4.2A motors = 16.8A / 70% = 11.76A so output any where around 11-12A will be fine. It's not critical as you'll never get anywhere near pulling the full 11A at one time.

GOLDEN RULE Don't buy anything unless your 110% sure it's correct for your needs.!! . . . . If not then ask on here first.!