I'd be more worried about accelerating the fat bloke, although it's still not going to change Robin's conclusion that a smaller motor is fine for your Z-axis.

The question really is do you envisage using the motors on a larger/heavier CNC Router, or a milling machine in the future? If so then it's sensible to get the 3Nm motors as they a very likely to be well matched to the future machine. If not then there are motors with a lower rated torque, but would perform better on your machine due to their lower inductance enabling them to reach a higher speed than the 3Nm motors. For instance play around with Irving's spreadsheet to compare this 1.85Nm 1.6mH one to the 3Nm motor for your size machine. The annoying thing then is you're spending more money on a smaller motor, so if they do turn out to be better in theory you've got to be sure you wont regret not having the bigger motors in the future for something else. Additionally the 3Nm motors are fit for purpose, so if they're cheaper than the better alternative then why not?

One point about the X-axis slaving - I wouldn't compare two motors and no belts to one motor and belts, since the belts have so many advantages anyway that you should use them with either system. Given that, out of your list of reasons, the only one that applies is you pretty much eliminate the chance of racking the gantry if the screws are linked. What are the chances of one motor stalling if you have two? Jazz said he's never had his stall since setting up with one motor and I've not had problems with two motors after tuning.

With both the motor selection and drive mechanism, in the end either system is fit for purpose so there's no point in me suggesting that you should use one or the other. What is worth suggesting though is if you do stick with trapezoidal screws, then consider substituting the nuts for the 'wonky bearing drive nut' method I started out using:

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Very low backlash, low friction and I bet you could make it with hand tools. There's certainly room for improvement on my design, some of which is discussed in my build log.

Hopefully I'm wrong, but it seems from your previous posts that you're considering not machining at least one end of the leadscrews presumably to save cost? If so forget it since the accuracy will be so poor.