I advise you read some of the build logs on this forum, as it is evident from the Z-axis design, some of the parts you have chosen and the question you ask, that you would do well to find out more before building this. For example, it's stronger to put the Z-axis rails on the same plate as the spindle and it would have been better to have RM1610 screws for X and Y.

20*330*160*2.7*10^-6=2.85kg, so either your scales are wrong or the dimensions of that plate are greater than specified.

3.1Nm motors will be plenty. Using timing belts and pulleys has numerous advantages, including getting better performance by changing the drive ratio, making it easier to mount the motor and helping with resonance damping. Since it seems you have bought 5mm pitch screws for X and Y, you will almost certainly need to use a 2:1 ratio (bigger pulley on motor) to make the machine run fast enough. Again, this information is nothing new - read a few build logs and you'll be able to answer the questions yourself.